Render maven's own reminders to a calendar she owns (#127)

Radicale becomes a write-only render target, not a store. sqlite stays
canonical: every poll mavcaldav reads the pending reminders out of core and
publishes each one as a single-event iCal resource, withdrawing the ones that
have fired or been cancelled. Losing the collection costs nothing — the next
tick rebuilds it, and nothing is ever read back from it.

It structurally cannot write to a calendar maven only reads. The render URL and
credential are their own flags, and -render-url is refused at startup when it
names the collection -url reads; the only paths it addresses carry the
maven-reminder- prefix, so even aimed at the wrong collection it can only touch
resources it created. Rendering is off unless -render-url is given.

The calendar data model now lives in one place, internal/calendar: the Event,
the iCal parse it comes from and the render it goes to, the fact key/value
encoding, and the source constants that say which calendars may be written to.
It was a parse inlined in cmd/mavcaldav and a Sprintf in two files; #126 and
#128 both need to agree with it.

Fixes a latent day-boundary bug moved out of that inline parse: it took the day
number off a local clock reading but built the window boundaries in UTC, so on
a box east of Greenwich part of the evening fell outside "today" and the poller
saw an empty calendar after 20:00 UTC. Today is now the owner's day in the
owner's location, which is what the busy gate and the day plan mean.

cmd/mavcaldav/main.go

@@ -1,17 +1,24 @@
-// mavcaldav — the CalDAV poller module.
+// mavcaldav — the CalDAV module: reads calendars into facts, and renders
+// maven's own reminders back out to a calendar she owns.
 //
-// Polls a Radicale (or any CalDAV) server for today's events and writes
-// `facts (kind=env, source=poll:caldav)` through core's IPC socket.
-// Key-free, restart-free, fail-independent — crashes can't touch the
-// store key, worst case a stale calendar_busy fact until the next poll.
+// READ side (unchanged behaviour): polls a Radicale (or any CalDAV) server for
+// today's events and writes `facts (kind=env, source=poll:caldav)` through
+// core's IPC socket. Key-free, restart-free, fail-independent — crashes can't
+// touch the store key, worst case a stale calendar_busy fact until the next
+// poll. Two facts:
 //
-// Two facts written:
 //   - calendar_busy  ("true"/"false") — read by the loop gate to suppress
 //     nudges during meetings
 //   - calendar_event ("<summary> @ <start>-<end>") — per-event for query
 //
-// Append-only discipline: a fact is written only when its value CHANGED
-// vs the latest for that key+source.
+// Append-only discipline: a fact is written only when its value CHANGED vs the
+// latest for that key+source.
+//
+// RENDER side (Vikunja #127, off unless -render-url is given): publishes each
+// pending reminder as a single-event iCal resource in a collection maven owns.
+// The calendar is a view, sqlite is the store — see render.go. The render URL
+// must differ from the read URL, checked at startup, so the render target can
+// never be a calendar maven is only supposed to read.
 package main
 
 import (
@@ -27,6 +34,7 @@ import (
 	"syscall"
 	"time"
 
+	"github.com/kami/maven/internal/calendar"
 	"github.com/kami/maven/internal/ipc"
 )
 
@@ -43,6 +51,10 @@ func run(args []string) error {
 	url := fs.String("url", "", "CalDAV calendar URL, e.g. http://localhost:5232/kami/personal (required)")
 	user := fs.String("user", "", "CalDAV basic-auth username (required)")
 	pass := fs.String("pass", "", "CalDAV basic-auth password (required)")
+	renderURL := fs.String("render-url", "", "CalDAV collection maven publishes her own reminders to; empty disables rendering")
+	renderUser := fs.String("render-user", "", "basic-auth username for -render-url (defaults to -user)")
+	renderPass := fs.String("render-pass", "", "basic-auth password for -render-url (defaults to -pass)")
+	renderDur := fs.Duration("render-duration", calendar.DefaultReminderDuration, "how long a rendered reminder occupies")
 	interval := fs.Duration("interval", 5*time.Minute, "poll cadence")
 	timeout := fs.Duration("timeout", 10*time.Second, "per-request HTTP timeout")
 	if err := fs.Parse(args); err != nil {
@@ -54,6 +66,9 @@ func run(args []string) error {
 	if *url == "" || *user == "" || *pass == "" {
 		return fmt.Errorf("-url, -user, -pass are required")
 	}
+	if err := checkRenderTarget(*url, *renderURL); err != nil {
+		return err
+	}
 
 	ctx, stop := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)
 	defer stop()
@@ -64,16 +79,36 @@ func run(args []string) error {
 	}
 	defer core.Close()
 
+	hc := &http.Client{Timeout: *timeout}
 	p := &poller{
 		core: core,
-		http: &http.Client{Timeout: *timeout},
+		http: hc,
 		url:  strings.TrimRight(*url, "/"),
 		user: *user,
 		pass: *pass,
 	}
 
+	var rend *renderer
+	if *renderURL != "" {
+		ru, rp := *renderUser, *renderPass
+		if ru == "" {
+			ru = *user
+		}
+		if rp == "" {
+			rp = *pass
+		}
+		rend = newRenderer(core, hc, *renderURL, ru, rp, *renderDur)
+		log.Printf("mavcaldav: rendering reminders to %s", *renderURL)
+	}
+
 	log.Printf("mavcaldav: polling %s every %s", *url, *interval)
-	p.pollOnce(ctx) // fire immediately
+	tick := func() {
+		p.pollOnce(ctx)
+		if rend != nil {
+			rend.renderOnce(ctx)
+		}
+	}
+	tick() // fire immediately
 	t := time.NewTicker(*interval)
 	defer t.Stop()
 	for {
@@ -82,11 +117,30 @@ func run(args []string) error {
 			log.Printf("mavcaldav: bye")
 			return nil
 		case <-t.C:
-			p.pollOnce(ctx)
+			tick()
 		}
 	}
 }
 
+// checkRenderTarget refuses a render URL that is also a read URL. This is the
+// structural half of #127's "cannot write to your work calendar": the write
+// credential and the write URL are separate flags, and the one calendar maven
+// is known to only read is rejected as a target at startup rather than trusted
+// at runtime.
+func checkRenderTarget(readURL, renderURL string) error {
+	if renderURL == "" {
+		return nil
+	}
+	if sameCollection(readURL, renderURL) {
+		return fmt.Errorf("-render-url must differ from -url: maven renders into a calendar she owns, never into one she reads")
+	}
+	return nil
+}
+
+func sameCollection(a, b string) bool {
+	return strings.EqualFold(strings.TrimRight(a, "/"), strings.TrimRight(b, "/"))
+}
+
 type poller struct {
 	core ipc.CoreAPI
 	http *http.Client
@@ -95,12 +149,6 @@ type poller struct {
 	pass string
 }
 
-type icalEvent struct {
-	start   time.Time
-	end     time.Time
-	summary string
-}
-
 func (p *poller) pollOnce(ctx context.Context) {
 	now := time.Now()
 	events, err := p.fetchEvents(ctx, now)
@@ -109,38 +157,30 @@ func (p *poller) pollOnce(ctx context.Context) {
 		return
 	}
 
-	busy := false
-	for _, e := range events {
-		if !now.Before(e.start) && now.Before(e.end) {
-			busy = true
-			break
-		}
-	}
 	busyVal := "false"
-	if busy {
+	if calendar.Busy(events, now) {
 		busyVal = "true"
 	}
 
 	// Write calendar_busy on change.
-	if err := p.writeIfChanged(ctx, "calendar_busy", "poll:caldav", busyVal, now); err != nil {
+	if err := p.writeIfChanged(ctx, "calendar_busy", calendar.SourcePersonal, busyVal, now, 1.0); err != nil {
 		log.Printf("mavcaldav: write calendar_busy: %v", err)
 		return
 	}
 
-	// Write per-event facts (one per event, keyed by event summary + start).
+	// Write per-event facts (one per event, keyed by day + event summary).
 	// This lets the note RAG path answer "what's on my calendar" without
 	// reaching back to Radicale.
 	for _, e := range events {
-		val := fmt.Sprintf("%s @ %s-%s", e.summary, e.start.Format("15:04"), e.end.Format("15:04"))
-		eventKey := fmt.Sprintf("calendar_event_%s_%s", e.start.Format("20060102"), safeKey(e.summary))
-		if err := p.writeIfChanged(ctx, eventKey, "poll:caldav", val, e.start); err != nil {
-			log.Printf("mavcaldav: write %s: %v", eventKey, err)
+		key := calendar.FactKey(e)
+		if err := p.writeIfChanged(ctx, key, calendar.SourcePersonal, calendar.FactValue(e), e.Start, 1.0); err != nil {
+			log.Printf("mavcaldav: write %s: %v", key, err)
 		}
 	}
 }
 
 // fetchEvents GETs the calendar URL and parses VEVENTs from the iCal response.
-func (p *poller) fetchEvents(ctx context.Context, now time.Time) ([]icalEvent, error) {
+func (p *poller) fetchEvents(ctx context.Context, now time.Time) ([]calendar.Event, error) {
 	req, err := http.NewRequestWithContext(ctx, http.MethodGet, p.url, nil)
 	if err != nil {
 		return nil, err
@@ -162,120 +202,11 @@ func (p *poller) fetchEvents(ctx context.Context, now time.Time) ([]icalEvent, e
 		return nil, fmt.Errorf("GET %s: %s", p.url, resp.Status)
 	}
 
-	return parseICal(body, now), nil
-}
-
-// parseICal scans iCal text for VEVENT components. Returns events that overlap
-// with today (UTC day boundaries) to keep the response manageable.
-func parseICal(body []byte, now time.Time) []icalEvent {
-	todayStart := time.Date(now.Year(), now.Month(), now.Day(), 0, 0, 0, 0, time.UTC)
-	todayEnd := todayStart.AddDate(0, 0, 1)
-
-	var events []icalEvent
-	text := string(body)
-	for {
-		veventStart := strings.Index(text, "BEGIN:VEVENT")
-		if veventStart < 0 {
-			break
-		}
-		text = text[veventStart+len("BEGIN:VEVENT"):]
-		veventEnd := strings.Index(text, "END:VEVENT")
-		if veventEnd < 0 {
-			break
-		}
-		block := text[:veventEnd]
-		text = text[veventEnd+len("END:VEVENT"):]
-
-		e := parseVEVENT(block)
-		if e == nil {
-			continue
-		}
-		// Only keep events overlapping today.
-		if e.end.After(todayStart) && e.start.Before(todayEnd) {
-			events = append(events, *e)
-		}
-	}
-	return events
-}
-
-// parseVEVENT extracts start, end, summary from a VEVENT block.
-// Supports both UTC (DTEND:20260703T100000Z) and local (DTSTART;TZID=...:...)
-// formats. Returns nil for all-day events (no DTSTART/DTEND time component) or
-// parse failures.
-func parseVEVENT(block string) *icalEvent {
-	var e icalEvent
-	lines := strings.Split(block, "\n")
-	for _, line := range lines {
-		line = strings.TrimSpace(line)
-		switch {
-		case strings.HasPrefix(line, "DTSTART"):
-			if t, ok := parseDT(line); ok {
-				e.start = t
-			}
-		case strings.HasPrefix(line, "DTEND"):
-			if t, ok := parseDT(line); ok {
-				e.end = t
-			}
-		case strings.HasPrefix(line, "SUMMARY"):
-			if idx := strings.Index(line, ":"); idx >= 0 {
-				e.summary = strings.TrimSpace(line[idx+1:])
-			}
-		}
-	}
-	if e.start.IsZero() || e.end.IsZero() {
-		return nil
-	}
-	return &e
-}
-
-// parseDT parses a DTSTART/DTEND value. Supports:
-//   - UTC: DTEND:20260703T100000Z
-//   - Local: DTSTART;TZID=Europe/Moscow:20260703T130000
-//   - Value-date (all-day): DTSTART;VALUE=DATE:20260703 (returns zero time)
-func parseDT(line string) (time.Time, bool) {
-	if strings.Contains(line, "VALUE=DATE:") {
-		return time.Time{}, false // all-day, skip
-	}
-	idx := strings.LastIndex(line, ":")
-	if idx < 0 {
-		return time.Time{}, false
-	}
-	val := line[idx+1:]
-	val = strings.TrimSuffix(val, "Z")
-
-	// Try UTC first (has Z suffix, or ended in Z before TrimSuffix).
-	if strings.HasSuffix(line, "Z") {
-		t, err := time.Parse("20060102T150405", val)
-		if err != nil {
-			return time.Time{}, false
-		}
-		return t.UTC(), true
-	}
-
-	// Local time — treat as UTC for simplicity (CalDAV server and poller
-	// run in the same timezone; the gate only needs busy/not-busy accuracy).
-	t, err := time.Parse("20060102T150405", val)
-	if err != nil {
-		return time.Time{}, false
-	}
-	return t.UTC(), true
-}
-
-// safeKey makes an event summary safe to use as a fact key (alphanumeric + dash).
-func safeKey(s string) string {
-	var b strings.Builder
-	for _, r := range s {
-		if (r >= 'a' && r <= 'z') || (r >= 'A' && r <= 'Z') || (r >= '0' && r <= '9') || r == '-' {
-			b.WriteRune(r)
-		} else if r == ' ' || r == '_' {
-			b.WriteRune('-')
-		}
-	}
-	return b.String()
+	return calendar.ParseICalDay(body, now), nil
 }
 
 // writeIfChanged writes a fact only when the value differs from the latest.
-func (p *poller) writeIfChanged(ctx context.Context, key, source, val string, ts time.Time) error {
+func (p *poller) writeIfChanged(ctx context.Context, key, source, val string, ts time.Time, confidence float64) error {
 	prev, err := p.core.LatestFactBySource(ctx, key, source)
 	switch {
 	case err == nil && prev.Value == val:
@@ -289,7 +220,7 @@ func (p *poller) writeIfChanged(ctx context.Context, key, source, val string, ts
 		Key:        key,
 		Value:      val,
 		Source:     source,
-		Confidence: 1.0,
+		Confidence: confidence,
 	})
 	if err != nil {
 		return fmt.Errorf("write %s: %w", key, err)

cmd/mavcaldav/main_test.go

@@ -51,165 +51,6 @@ func (f *fakeCore) WriteFact(_ context.Context, req ipc.WriteFactReq) (int64, er
 	return int64(len(f.writeLog)), nil
 }
 
-// ---------------------------------------------------------------------------
-// Parsing tests
-// ---------------------------------------------------------------------------
-
-func TestParseICal(t *testing.T) {
-	now := time.Date(2026, 7, 3, 12, 0, 0, 0, time.UTC)
-
-	body := []byte(`BEGIN:VCALENDAR
-BEGIN:VEVENT
-DTSTART:20260703T090000Z
-DTEND:20260703T100000Z
-SUMMARY:Morning standup
-END:VEVENT
-BEGIN:VEVENT
-DTSTART:20260703T140000Z
-DTEND:20260703T150000Z
-SUMMARY:Team sync
-END:VEVENT
-BEGIN:VEVENT
-DTSTART:20260702T140000Z
-DTEND:20260702T150000Z
-SUMMARY:Yesterday retro
-END:VEVENT
-BEGIN:VEVENT
-DTSTART:20260704T090000Z
-DTEND:20260704T100000Z
-SUMMARY:Tomorrow standup
-END:VEVENT
-BEGIN:VEVENT
-DTSTART;VALUE=DATE:20260704
-DTEND;VALUE=DATE:20260705
-SUMMARY:All-day event
-END:VEVENT
-END:VCALENDAR`)
-
-	events := parseICal(body, now)
-
-	if len(events) != 2 {
-		t.Fatalf("got %d events, want 2 (today events, no all-day/past/future)", len(events))
-	}
-
-	// Morning standup — overlaps today.
-	if events[0].summary != "Morning standup" {
-		t.Errorf("events[0].summary = %q, want %q", events[0].summary, "Morning standup")
-	}
-	wantStart0 := time.Date(2026, 7, 3, 9, 0, 0, 0, time.UTC)
-	if !events[0].start.Equal(wantStart0) {
-		t.Errorf("events[0].start = %v, want %v", events[0].start, wantStart0)
-	}
-	wantEnd0 := time.Date(2026, 7, 3, 10, 0, 0, 0, time.UTC)
-	if !events[0].end.Equal(wantEnd0) {
-		t.Errorf("events[0].end = %v, want %v", events[0].end, wantEnd0)
-	}
-
-	// Team sync — overlaps today.
-	if events[1].summary != "Team sync" {
-		t.Errorf("events[1].summary = %q, want %q", events[1].summary, "Team sync")
-	}
-	wantStart1 := time.Date(2026, 7, 3, 14, 0, 0, 0, time.UTC)
-	if !events[1].start.Equal(wantStart1) {
-		t.Errorf("events[1].start = %v, want %v", events[1].start, wantStart1)
-	}
-	wantEnd1 := time.Date(2026, 7, 3, 15, 0, 0, 0, time.UTC)
-	if !events[1].end.Equal(wantEnd1) {
-		t.Errorf("events[1].end = %v, want %v", events[1].end, wantEnd1)
-	}
-}
-
-func TestParseVEVENT(t *testing.T) {
-	// Normal event with TZID in DTSTART and UTC DTEND.
-	block := "DTSTART;TZID=Europe/Moscow:20260703T130000\nDTEND:20260703T140000Z\nSUMMARY:Stand up meeting"
-	e := parseVEVENT(block)
-	if e == nil {
-		t.Fatal("expected non-nil icalEvent")
-	}
-	wantStart := time.Date(2026, 7, 3, 13, 0, 0, 0, time.UTC)
-	if !e.start.Equal(wantStart) {
-		t.Errorf("start = %v, want %v", e.start, wantStart)
-	}
-	wantEnd := time.Date(2026, 7, 3, 14, 0, 0, 0, time.UTC)
-	if !e.end.Equal(wantEnd) {
-		t.Errorf("end = %v, want %v", e.end, wantEnd)
-	}
-	if e.summary != "Stand up meeting" {
-		t.Errorf("summary = %q, want %q", e.summary, "Stand up meeting")
-	}
-
-	// All-day event (VALUE=DATE) → nil.
-	allDay := "DTSTART;VALUE=DATE:20260703\nDTEND;VALUE=DATE:20260704\nSUMMARY:All-day"
-	if e2 := parseVEVENT(allDay); e2 != nil {
-		t.Error("expected nil for all-day event")
-	}
-}
-
-func TestParseDT(t *testing.T) {
-	tests := []struct {
-		name   string
-		line   string
-		want   time.Time
-		wantOK bool
-	}{
-		{
-			name:   "UTC",
-			line:   "DTEND:20260703T100000Z",
-			want:   time.Date(2026, 7, 3, 10, 0, 0, 0, time.UTC),
-			wantOK: true,
-		},
-		{
-			name:   "local time",
-			line:   "DTSTART;TZID=Europe/Moscow:20260703T130000",
-			want:   time.Date(2026, 7, 3, 13, 0, 0, 0, time.UTC),
-			wantOK: true,
-		},
-		{
-			name:   "all-day",
-			line:   "DTSTART;VALUE=DATE:20260703",
-			want:   time.Time{},
-			wantOK: false,
-		},
-		{
-			name:   "invalid",
-			line:   "DTSTART:garbage",
-			want:   time.Time{},
-			wantOK: false,
-		},
-	}
-
-	for _, tt := range tests {
-		t.Run(tt.name, func(t *testing.T) {
-			got, ok := parseDT(tt.line)
-			if ok != tt.wantOK {
-				t.Errorf("ok = %v, want %v", ok, tt.wantOK)
-			}
-			if !got.Equal(tt.want) {
-				t.Errorf("got = %v, want %v", got, tt.want)
-			}
-		})
-	}
-}
-
-func TestSafeKey(t *testing.T) {
-	tests := []struct {
-		input string
-		want  string
-	}{
-		{"Stand up meeting", "Stand-up-meeting"},
-		{"Hello_World", "Hello-World"},
-		{"special@#$chars!!", "specialchars"},
-		{"ALL_CAPS_123", "ALL-CAPS-123"},
-	}
-
-	for _, tt := range tests {
-		got := safeKey(tt.input)
-		if got != tt.want {
-			t.Errorf("safeKey(%q) = %q, want %q", tt.input, got, tt.want)
-		}
-	}
-}
-
 // ---------------------------------------------------------------------------
 // Core logic tests
 // ---------------------------------------------------------------------------
@@ -221,7 +62,7 @@ func TestWriteIfChanged(t *testing.T) {
 	t.Run("no previous fact writes", func(t *testing.T) {
 		fc := &fakeCore{}
 		p := &poller{core: fc}
-		err := p.writeIfChanged(ctx, "test_key", "poll:caldav", "hello", now)
+		err := p.writeIfChanged(ctx, "test_key", "poll:caldav", "hello", now, 1.0)
 		if err != nil {
 			t.Fatalf("unexpected error: %v", err)
 		}
@@ -249,7 +90,7 @@ func TestWriteIfChanged(t *testing.T) {
 			},
 		}
 		p := &poller{core: fc}
-		err := p.writeIfChanged(ctx, "test_key", "poll:caldav", "hello", now)
+		err := p.writeIfChanged(ctx, "test_key", "poll:caldav", "hello", now, 1.0)
 		if err != nil {
 			t.Fatalf("unexpected error: %v", err)
 		}
@@ -265,7 +106,7 @@ func TestWriteIfChanged(t *testing.T) {
 			},
 		}
 		p := &poller{core: fc}
-		err := p.writeIfChanged(ctx, "test_key", "poll:caldav", "new", now)
+		err := p.writeIfChanged(ctx, "test_key", "poll:caldav", "new", now, 1.0)
 		if err != nil {
 			t.Fatalf("unexpected error: %v", err)
 		}
@@ -280,7 +121,7 @@ func TestWriteIfChanged(t *testing.T) {
 	t.Run("read error other than ErrNoFact returns error", func(t *testing.T) {
 		fc := &fakeCore{readErr: fmt.Errorf("connection refused")}
 		p := &poller{core: fc}
-		err := p.writeIfChanged(ctx, "fail_key", "poll:caldav", "x", now)
+		err := p.writeIfChanged(ctx, "fail_key", "poll:caldav", "x", now, 1.0)
 		if err == nil {
 			t.Fatal("expected error, got nil")
 		}
@@ -292,7 +133,7 @@ func TestWriteIfChanged(t *testing.T) {
 			writeErr: fmt.Errorf("disk full"),
 		}
 		p := &poller{core: fc}
-		err := p.writeIfChanged(ctx, "test_key", "poll:caldav", "hello", now)
+		err := p.writeIfChanged(ctx, "test_key", "poll:caldav", "hello", now, 1.0)
 		if err == nil {
 			t.Fatal("expected error, got nil")
 		}

cmd/mavcaldav/render.go

@@ -0,0 +1,145 @@
+package main
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"log"
+	"net/http"
+	"strings"
+	"time"
+
+	"github.com/kami/maven/internal/calendar"
+	"github.com/kami/maven/internal/ipc"
+)
+
+// renderer is the write half of maven's own local calendar (Vikunja #127).
+//
+// It is a RENDER TARGET, not a store. sqlite stays canonical: every tick the
+// renderer reads the pending reminders out of core and publishes each one as a
+// single-event iCal resource in a CalDAV collection maven owns. Nothing is ever
+// read back from that collection, and losing it costs nothing — the next tick
+// rebuilds it.
+//
+// It structurally cannot write to a calendar maven only reads. The URL comes
+// from its own flag, checked at startup against every read URL (see
+// run in main.go), and the only paths it ever addresses carry
+// calendar.ReminderUIDPrefix — so even pointed at the wrong collection it can
+// only touch resources it created.
+type renderer struct {
+	core ipc.CoreAPI
+	http *http.Client
+	url  string
+	user string
+	pass string
+	dur  time.Duration
+
+	// published maps reminder id → the body last successfully PUT, so an
+	// unchanged reminder costs nothing. Purely an optimisation: a restart
+	// re-publishes every reminder once, which is idempotent.
+	published map[int64]string
+}
+
+func newRenderer(core ipc.CoreAPI, hc *http.Client, url, user, pass string, dur time.Duration) *renderer {
+	return &renderer{
+		core:      core,
+		http:      hc,
+		url:       strings.TrimRight(url, "/"),
+		user:      user,
+		pass:      pass,
+		dur:       dur,
+		published: make(map[int64]string),
+	}
+}
+
+// renderOnce publishes every pending reminder and withdraws the ones that are
+// no longer pending. Errors are logged and skipped: a calendar maven cannot
+// reach must never break the reminder itself, which lives in sqlite.
+func (r *renderer) renderOnce(ctx context.Context) {
+	reminders, err := r.core.ListReminders(ctx, renderMaxReminders)
+	if err != nil {
+		log.Printf("mavcaldav: list reminders: %v", err)
+		return
+	}
+
+	live := make(map[int64]bool, len(reminders))
+	for _, rem := range reminders {
+		if rem.Status != "pending" {
+			continue
+		}
+		live[rem.ID] = true
+		e := calendar.ReminderEvent(rem.ID, fireTime(rem), rem.Payload, r.dur)
+		body := calendar.RenderICal([]calendar.Event{e})
+		if r.published[rem.ID] == body {
+			continue
+		}
+		if err := r.put(ctx, calendar.ReminderPath(rem.ID), body); err != nil {
+			log.Printf("mavcaldav: render reminder %d: %v", rem.ID, err)
+			continue
+		}
+		r.published[rem.ID] = body
+		log.Printf("mavcaldav: rendered reminder %d (%s)", rem.ID, e.Summary)
+	}
+
+	for id := range r.published {
+		if live[id] {
+			continue
+		}
+		if err := r.delete(ctx, calendar.ReminderPath(id)); err != nil {
+			log.Printf("mavcaldav: withdraw reminder %d: %v", id, err)
+			continue
+		}
+		delete(r.published, id)
+		log.Printf("mavcaldav: withdrew reminder %d", id)
+	}
+}
+
+// renderMaxReminders bounds the read. Reminders past this count are older than
+// anything a calendar view is useful for.
+const renderMaxReminders = 200
+
+// fireTime prefers NextFireTs — for a recurring reminder that is the occurrence
+// worth showing; FireTs is the original statement.
+func fireTime(rem ipc.Reminder) time.Time {
+	if !rem.NextFireTs.IsZero() {
+		return rem.NextFireTs
+	}
+	return rem.FireTs
+}
+
+func (r *renderer) put(ctx context.Context, name, body string) error {
+	req, err := http.NewRequestWithContext(ctx, http.MethodPut, r.url+"/"+name, strings.NewReader(body))
+	if err != nil {
+		return err
+	}
+	req.SetBasicAuth(r.user, r.pass)
+	req.Header.Set("Content-Type", "text/calendar; charset=utf-8")
+	return r.do(req, name)
+}
+
+func (r *renderer) delete(ctx context.Context, name string) error {
+	req, err := http.NewRequestWithContext(ctx, http.MethodDelete, r.url+"/"+name, nil)
+	if err != nil {
+		return err
+	}
+	req.SetBasicAuth(r.user, r.pass)
+	return r.do(req, name)
+}
+
+// do runs the request and treats any 2xx, plus 404 on a DELETE, as success —
+// a resource that is already gone is the state the caller wanted.
+func (r *renderer) do(req *http.Request, name string) error {
+	resp, err := r.http.Do(req)
+	if err != nil {
+		return err
+	}
+	defer resp.Body.Close()
+	io.Copy(io.Discard, io.LimitReader(resp.Body, 1<<16))
+	switch {
+	case resp.StatusCode >= 200 && resp.StatusCode < 300:
+		return nil
+	case req.Method == http.MethodDelete && resp.StatusCode == http.StatusNotFound:
+		return nil
+	}
+	return fmt.Errorf("%s %s: %s", req.Method, name, resp.Status)
+}

cmd/mavcaldav/render_test.go

@@ -0,0 +1,186 @@
+package main
+
+import (
+	"context"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/kami/maven/internal/ipc"
+)
+
+// reminderCore is a fakeCore that also answers ListReminders.
+type reminderCore struct {
+	fakeCore
+	reminders []ipc.Reminder
+	listErr   error
+}
+
+func (c *reminderCore) ListReminders(context.Context, int) ([]ipc.Reminder, error) {
+	if c.listErr != nil {
+		return nil, c.listErr
+	}
+	return c.reminders, nil
+}
+
+// calSrv records what a CalDAV collection received.
+type calSrv struct {
+	mu     sync.Mutex
+	puts   map[string]string
+	dels   []string
+	status int
+	*httptest.Server
+}
+
+func newCalSrv() *calSrv {
+	s := &calSrv{puts: map[string]string{}, status: http.StatusCreated}
+	s.Server = httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		body, _ := io.ReadAll(r.Body)
+		s.mu.Lock()
+		defer s.mu.Unlock()
+		switch r.Method {
+		case http.MethodPut:
+			s.puts[strings.TrimPrefix(r.URL.Path, "/cal/")] = string(body)
+		case http.MethodDelete:
+			s.dels = append(s.dels, strings.TrimPrefix(r.URL.Path, "/cal/"))
+		}
+		w.WriteHeader(s.status)
+	}))
+	return s
+}
+
+func (s *calSrv) putCount() int {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	return len(s.puts)
+}
+
+func TestRenderOncePublishesPendingReminders(t *testing.T) {
+	fire := time.Date(2026, 8, 1, 18, 30, 0, 0, time.UTC)
+	srv := newCalSrv()
+	defer srv.Close()
+
+	core := &reminderCore{reminders: []ipc.Reminder{
+		{ID: 7, FireTs: fire, Payload: "позвонить маме", Status: "pending"},
+		{ID: 8, FireTs: fire, Payload: "уже сделано", Status: "fired"},
+		{ID: 9, FireTs: fire, Payload: "отменено", Status: "cancelled"},
+	}}
+	r := newRenderer(core, srv.Client(), srv.URL+"/cal/", "u", "p", 0)
+	r.renderOnce(context.Background())
+
+	srv.mu.Lock()
+	body, ok := srv.puts["maven-reminder-7.ics"]
+	n := len(srv.puts)
+	srv.mu.Unlock()
+
+	if n != 1 {
+		t.Fatalf("expected exactly the pending reminder to be published, got %d PUTs", n)
+	}
+	if !ok {
+		t.Fatal("pending reminder 7 was not published")
+	}
+	if !strings.Contains(body, "SUMMARY:позвонить маме") {
+		t.Errorf("payload missing from rendered body:\n%s", body)
+	}
+	if !strings.Contains(body, "UID:maven-reminder-7") {
+		t.Errorf("UID missing from rendered body:\n%s", body)
+	}
+}
+
+func TestRenderOnceSkipsUnchanged(t *testing.T) {
+	srv := newCalSrv()
+	defer srv.Close()
+	core := &reminderCore{reminders: []ipc.Reminder{
+		{ID: 1, FireTs: time.Date(2026, 8, 1, 9, 0, 0, 0, time.UTC), Payload: "выпить воды", Status: "pending"},
+	}}
+	r := newRenderer(core, srv.Client(), srv.URL+"/cal", "u", "p", 0)
+	r.renderOnce(context.Background())
+	r.renderOnce(context.Background())
+	if got := srv.putCount(); got != 1 {
+		t.Fatalf("an unchanged reminder was re-published: %d distinct PUTs", got)
+	}
+}
+
+func TestRenderOnceWithdrawsResolvedReminders(t *testing.T) {
+	srv := newCalSrv()
+	defer srv.Close()
+	core := &reminderCore{reminders: []ipc.Reminder{
+		{ID: 5, FireTs: time.Date(2026, 8, 1, 9, 0, 0, 0, time.UTC), Payload: "встреча", Status: "pending"},
+	}}
+	r := newRenderer(core, srv.Client(), srv.URL+"/cal", "u", "p", 0)
+	r.renderOnce(context.Background())
+
+	core.reminders[0].Status = "fired"
+	r.renderOnce(context.Background())
+
+	srv.mu.Lock()
+	dels := append([]string(nil), srv.dels...)
+	srv.mu.Unlock()
+	if len(dels) != 1 || dels[0] != "maven-reminder-5.ics" {
+		t.Fatalf("resolved reminder was not withdrawn: %v", dels)
+	}
+	if len(r.published) != 0 {
+		t.Errorf("published map still holds %v", r.published)
+	}
+}
+
+// A calendar maven cannot reach must never break anything: sqlite is canonical.
+func TestRenderOnceSurvivesServerErrors(t *testing.T) {
+	srv := newCalSrv()
+	srv.status = http.StatusInternalServerError
+	defer srv.Close()
+	core := &reminderCore{reminders: []ipc.Reminder{
+		{ID: 1, FireTs: time.Date(2026, 8, 1, 9, 0, 0, 0, time.UTC), Payload: "x", Status: "pending"},
+	}}
+	r := newRenderer(core, srv.Client(), srv.URL+"/cal", "u", "p", 0)
+	r.renderOnce(context.Background())
+	if len(r.published) != 0 {
+		t.Error("a failed PUT must not be recorded as published, or it never retries")
+	}
+}
+
+func TestRenderOnceUsesNextFireForRecurring(t *testing.T) {
+	srv := newCalSrv()
+	defer srv.Close()
+	next := time.Date(2026, 8, 2, 7, 0, 0, 0, time.UTC)
+	core := &reminderCore{reminders: []ipc.Reminder{{
+		ID:         3,
+		FireTs:     time.Date(2026, 8, 1, 7, 0, 0, 0, time.UTC),
+		NextFireTs: next,
+		Payload:    "зарядка",
+		Status:     "pending",
+		Cron:       "0 7 * * *",
+	}}}
+	r := newRenderer(core, srv.Client(), srv.URL+"/cal", "u", "p", 0)
+	r.renderOnce(context.Background())
+
+	srv.mu.Lock()
+	body := srv.puts["maven-reminder-3.ics"]
+	srv.mu.Unlock()
+	if !strings.Contains(body, "DTSTART:20260802T070000Z") {
+		t.Errorf("recurring reminder should render its next occurrence:\n%s", body)
+	}
+}
+
+func TestCheckRenderTargetRefusesTheCalendarItReads(t *testing.T) {
+	read := "http://localhost:5232/kami/personal"
+	if err := checkRenderTarget(read, ""); err != nil {
+		t.Fatalf("rendering off must be fine: %v", err)
+	}
+	if err := checkRenderTarget(read, "http://localhost:5232/kami/maven"); err != nil {
+		t.Fatalf("a distinct collection must be accepted: %v", err)
+	}
+	if err := checkRenderTarget(read, read); err == nil {
+		t.Error("rendering into the read calendar must be refused")
+	}
+	if err := checkRenderTarget(read, read+"/"); err == nil {
+		t.Error("a trailing slash must not defeat the check")
+	}
+	if err := checkRenderTarget(read, strings.ToUpper(read)); err == nil {
+		t.Error("case must not defeat the check")
+	}
+}

internal/calendar/calendar.go

@@ -0,0 +1,129 @@
+// Package calendar is the one calendar data model the rest of maven shares:
+// an Event, the iCal text it is parsed from and rendered to, and the fact
+// encoding that puts it in the store.
+//
+// It exists because three separate features read or write the same events and
+// must agree on their shape: the CalDAV read side (cmd/mavcaldav, Vikunja
+// #126/#127), the write-only render target that publishes maven's own
+// reminders as a calendar (#127), and the day plan that recites them (#128).
+// Before this package the parse lived inline in cmd/mavcaldav and the fact key
+// format was a Sprintf in two places.
+//
+// The package is pure: no HTTP, no store, no clock of its own. Callers own the
+// impurity, the way internal/morning and internal/loop do.
+package calendar
+
+import (
+	"fmt"
+	"sort"
+	"strings"
+	"time"
+)
+
+// Fact sources. A calendar event reaches the store as a
+// `facts (kind=env, key=calendar_event_..., source=<one of these>)` row, and
+// the source is the whole provenance story:
+//
+//   - SourcePersonal — maven's own Radicale, read AND rendered to. Canonical
+//     state stays in sqlite; the calendar is a render target (#127).
+//   - SourceWork — a work calendar, read-only by definition (#126). Nothing in
+//     maven ever writes to it: no code path pairs this source with a PUT.
+//   - SourceAmbient — inferred from an Android notification-listener relay
+//     rather than read from a server (#126). Confidence is below 1.0 because a
+//     notification is a signal about an event, not the event.
+const (
+	SourcePersonal = "poll:caldav"
+	SourceWork     = "poll:caldav:work"
+	SourceAmbient  = "ambient:notif"
+)
+
+// AmbientConfidence — the confidence a notification-derived event is stored
+// with. A parsed notification line is evidence, not a reading of the calendar,
+// so it must never be indistinguishable from one (#126).
+const AmbientConfidence = 0.6
+
+// Sources lists every source a calendar event may legitimately carry, for the
+// store query that reads the calendar back out. Ordered from most to least
+// trusted.
+func Sources() []string {
+	return []string{SourcePersonal, SourceWork, SourceAmbient}
+}
+
+// ReadOnlySource reports whether events from this source may never be written
+// back. The work calendar is read-only by definition — see #126: maven holding
+// a credential that can write to an employer's calendar is the thing the task
+// exists to avoid.
+func ReadOnlySource(source string) bool {
+	return source == SourceWork || source == SourceAmbient
+}
+
+// Event — one calendar entry. UID is the iCal UID when the event was parsed
+// from a server and the identity maven renders under when it publishes one;
+// Start/End are instants. All-day events are not modelled: the busy gate and
+// the day plan both need a time of day, and an all-day marker answers neither.
+type Event struct {
+	UID     string
+	Summary string
+	Start   time.Time
+	End     time.Time
+}
+
+// FactKey is the store key for an event: one key per day per summary, stable
+// across polls so re-reading an unchanged calendar rewrites nothing.
+//
+// The date prefix is load-bearing — store.CalendarEvents selects a day range
+// by key prefix, not by a timestamp column.
+func FactKey(e Event) string {
+	return fmt.Sprintf("calendar_event_%s_%s", e.Start.Format("20060102"), safeKey(e.Summary))
+}
+
+// FactValue is the human-readable rendering stored as the fact value, and the
+// string the day plan and the query path read back.
+func FactValue(e Event) string {
+	return fmt.Sprintf("%s @ %s-%s", e.Summary, e.Start.Format("15:04"), e.End.Format("15:04"))
+}
+
+// KeyPrefixForDay is the fact-key prefix covering one calendar day. The store
+// range-scans between two of these.
+func KeyPrefixForDay(day time.Time) string {
+	return fmt.Sprintf("calendar_event_%s", day.Format("20060102"))
+}
+
+// Busy reports whether any event covers the instant now — the read the loop
+// gate uses to suppress nudges during a meeting.
+func Busy(events []Event, now time.Time) bool {
+	for _, e := range events {
+		if !now.Before(e.Start) && now.Before(e.End) {
+			return true
+		}
+	}
+	return false
+}
+
+// Overlapping returns the events intersecting [from, to), sorted by start.
+func Overlapping(events []Event, from, to time.Time) []Event {
+	var out []Event
+	for _, e := range events {
+		if e.End.After(from) && e.Start.Before(to) {
+			out = append(out, e)
+		}
+	}
+	sort.Slice(out, func(i, j int) bool { return out[i].Start.Before(out[j].Start) })
+	return out
+}
+
+// safeKey makes a summary safe to use inside a fact key (ASCII alphanumerics
+// and dashes). Non-Latin summaries collapse to their punctuation, which is why
+// the day prefix carries the identity and this only disambiguates within a day.
+func safeKey(s string) string {
+	var b strings.Builder
+	for _, r := range s {
+		switch {
+		case (r >= 'a' && r <= 'z') || (r >= 'A' && r <= 'Z') || (r >= '0' && r <= '9') || r == '-':
+			b.WriteRune(r)
+		case r == ' ' || r == '_':
+			b.WriteRune('-')
+		}
+	}
+	return b.String()
+}

internal/calendar/calendar_test.go

@@ -0,0 +1,202 @@
+package calendar
+
+import (
+	"strings"
+	"testing"
+	"time"
+)
+
+func TestParseICalDayKeepsOnlyToday(t *testing.T) {
+	now := time.Date(2026, 7, 3, 12, 0, 0, 0, time.UTC)
+
+	body := []byte(`BEGIN:VCALENDAR
+BEGIN:VEVENT
+UID:a@example
+DTSTART:20260703T090000Z
+DTEND:20260703T100000Z
+SUMMARY:Morning standup
+END:VEVENT
+BEGIN:VEVENT
+DTSTART:20260703T140000Z
+DTEND:20260703T150000Z
+SUMMARY:Team sync
+END:VEVENT
+BEGIN:VEVENT
+DTSTART:20260702T140000Z
+DTEND:20260702T150000Z
+SUMMARY:Yesterday retro
+END:VEVENT
+BEGIN:VEVENT
+DTSTART:20260704T090000Z
+DTEND:20260704T100000Z
+SUMMARY:Tomorrow standup
+END:VEVENT
+BEGIN:VEVENT
+DTSTART;VALUE=DATE:20260704
+DTEND;VALUE=DATE:20260705
+SUMMARY:All-day event
+END:VEVENT
+END:VCALENDAR`)
+
+	events := ParseICalDay(body, now)
+	if len(events) != 2 {
+		t.Fatalf("got %d events, want 2 (today only, no all-day/past/future)", len(events))
+	}
+	if events[0].Summary != "Morning standup" || events[0].UID != "a@example" {
+		t.Errorf("events[0] = %+v", events[0])
+	}
+	if !events[0].Start.Equal(time.Date(2026, 7, 3, 9, 0, 0, 0, time.UTC)) {
+		t.Errorf("events[0].Start = %v", events[0].Start)
+	}
+	if !events[0].End.Equal(time.Date(2026, 7, 3, 10, 0, 0, 0, time.UTC)) {
+		t.Errorf("events[0].End = %v", events[0].End)
+	}
+	if events[1].Summary != "Team sync" {
+		t.Errorf("events[1].Summary = %q", events[1].Summary)
+	}
+}
+
+// Regression: "today" is the owner's day, in the owner's location. Taking the
+// day number off a local clock but building the boundaries in UTC made the
+// evening fall outside the window on any box east of Greenwich.
+func TestParseICalDayUsesOwnersDay(t *testing.T) {
+	plus4 := time.FixedZone("+04", 4*60*60)
+	// 01:00 on Aug 1 local is 21:00 on Jul 31 UTC.
+	now := time.Date(2026, 8, 1, 1, 0, 0, 0, plus4)
+	body := []byte("BEGIN:VCALENDAR\nBEGIN:VEVENT\n" +
+		"DTSTART:20260731T195406Z\nDTEND:20260731T235406Z\nSUMMARY:Current meeting\n" +
+		"END:VEVENT\nEND:VCALENDAR")
+
+	events := ParseICalDay(body, now)
+	if len(events) != 1 {
+		t.Fatalf("got %d events, want the in-progress one", len(events))
+	}
+	if !Busy(events, now.UTC()) {
+		t.Error("an event in progress right now must read as busy")
+	}
+}
+
+func TestParseVEVENT(t *testing.T) {
+	block := "DTSTART;TZID=Europe/Moscow:20260703T130000\nDTEND:20260703T140000Z\nSUMMARY:Stand up meeting"
+	e, ok := parseVEVENT(block)
+	if !ok {
+		t.Fatal("expected a parsed event")
+	}
+	if !e.Start.Equal(time.Date(2026, 7, 3, 13, 0, 0, 0, time.UTC)) {
+		t.Errorf("start = %v", e.Start)
+	}
+	if !e.End.Equal(time.Date(2026, 7, 3, 14, 0, 0, 0, time.UTC)) {
+		t.Errorf("end = %v", e.End)
+	}
+	if e.Summary != "Stand up meeting" {
+		t.Errorf("summary = %q", e.Summary)
+	}
+
+	allDay := "DTSTART;VALUE=DATE:20260703\nDTEND;VALUE=DATE:20260704\nSUMMARY:All-day"
+	if _, ok := parseVEVENT(allDay); ok {
+		t.Error("all-day event should be rejected")
+	}
+}
+
+func TestParseDT(t *testing.T) {
+	tests := []struct {
+		name   string
+		line   string
+		want   time.Time
+		wantOK bool
+	}{
+		{"UTC", "DTEND:20260703T100000Z", time.Date(2026, 7, 3, 10, 0, 0, 0, time.UTC), true},
+		{"local", "DTSTART;TZID=Europe/Moscow:20260703T130000", time.Date(2026, 7, 3, 13, 0, 0, 0, time.UTC), true},
+		{"all-day", "DTSTART;VALUE=DATE:20260703", time.Time{}, false},
+		{"garbage", "DTSTART:garbage", time.Time{}, false},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, ok := parseDT(tt.line)
+			if ok != tt.wantOK {
+				t.Errorf("ok = %v, want %v", ok, tt.wantOK)
+			}
+			if !got.Equal(tt.want) {
+				t.Errorf("got %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
+
+func TestSafeKey(t *testing.T) {
+	tests := []struct{ in, want string }{
+		{"Stand up meeting", "Stand-up-meeting"},
+		{"Hello_World", "Hello-World"},
+		{"special@#$chars!!", "specialchars"},
+		{"ALL_CAPS_123", "ALL-CAPS-123"},
+	}
+	for _, tt := range tests {
+		if got := safeKey(tt.in); got != tt.want {
+			t.Errorf("safeKey(%q) = %q, want %q", tt.in, got, tt.want)
+		}
+	}
+}
+
+func TestFactKeyAndValue(t *testing.T) {
+	e := Event{
+		Summary: "Team sync",
+		Start:   time.Date(2026, 7, 3, 14, 0, 0, 0, time.UTC),
+		End:     time.Date(2026, 7, 3, 15, 0, 0, 0, time.UTC),
+	}
+	if got, want := FactKey(e), "calendar_event_20260703_Team-sync"; got != want {
+		t.Errorf("FactKey = %q, want %q", got, want)
+	}
+	if got, want := FactValue(e), "Team sync @ 14:00-15:00"; got != want {
+		t.Errorf("FactValue = %q, want %q", got, want)
+	}
+	if got, want := KeyPrefixForDay(e.Start), "calendar_event_20260703"; got != want {
+		t.Errorf("KeyPrefixForDay = %q, want %q", got, want)
+	}
+	if !strings.HasPrefix(FactKey(e), KeyPrefixForDay(e.Start)) {
+		t.Error("FactKey must start with the day prefix the store range-scans on")
+	}
+}
+
+func TestBusyAndOverlapping(t *testing.T) {
+	base := time.Date(2026, 7, 3, 0, 0, 0, 0, time.UTC)
+	events := []Event{
+		{Summary: "late", Start: base.Add(15 * time.Hour), End: base.Add(16 * time.Hour)},
+		{Summary: "early", Start: base.Add(9 * time.Hour), End: base.Add(10 * time.Hour)},
+	}
+	if !Busy(events, base.Add(9*time.Hour+30*time.Minute)) {
+		t.Error("should be busy inside the early event")
+	}
+	if Busy(events, base.Add(12*time.Hour)) {
+		t.Error("should be free at noon")
+	}
+	// Half-open: the end instant is free.
+	if Busy(events, base.Add(10*time.Hour)) {
+		t.Error("the end instant should not count as busy")
+	}
+	got := Overlapping(events, base.Add(8*time.Hour), base.Add(11*time.Hour))
+	if len(got) != 1 || got[0].Summary != "early" {
+		t.Fatalf("Overlapping = %+v", got)
+	}
+	all := Overlapping(events, base, base.AddDate(0, 0, 1))
+	if len(all) != 2 || all[0].Summary != "early" {
+		t.Fatalf("Overlapping must sort by start: %+v", all)
+	}
+}
+
+func TestSourceTrust(t *testing.T) {
+	if ReadOnlySource(SourcePersonal) {
+		t.Error("the personal calendar is the one maven may render to")
+	}
+	if !ReadOnlySource(SourceWork) {
+		t.Error("the work calendar must be read-only")
+	}
+	if !ReadOnlySource(SourceAmbient) {
+		t.Error("an ambient notification is not a writable calendar")
+	}
+	if AmbientConfidence >= 1.0 {
+		t.Error("ambient events must be less trusted than a calendar read")
+	}
+	if len(Sources()) != 3 {
+		t.Errorf("Sources() = %v", Sources())
+	}
+}

internal/calendar/ical.go

@@ -0,0 +1,145 @@
+package calendar
+
+import (
+	"fmt"
+	"strings"
+	"time"
+)
+
+// ParseICal scans iCal text for VEVENT components and returns the events
+// overlapping [from, to). All-day events are skipped: parseDT reports no time
+// for a VALUE=DATE value, and an event with no clock reading answers neither
+// the busy gate nor the day plan.
+func ParseICal(body []byte, from, to time.Time) []Event {
+	var events []Event
+	text := string(body)
+	for {
+		i := strings.Index(text, "BEGIN:VEVENT")
+		if i < 0 {
+			break
+		}
+		text = text[i+len("BEGIN:VEVENT"):]
+		j := strings.Index(text, "END:VEVENT")
+		if j < 0 {
+			break
+		}
+		block := text[:j]
+		text = text[j+len("END:VEVENT"):]
+
+		e, ok := parseVEVENT(block)
+		if !ok {
+			continue
+		}
+		if e.End.After(from) && e.Start.Before(to) {
+			events = append(events, e)
+		}
+	}
+	return events
+}
+
+// ParseICalDay is ParseICal over the calendar day containing now, in now's own
+// location — the window cmd/mavcaldav polls.
+//
+// The location matters. The old inline version took the day number off a local
+// clock reading but built the boundaries in UTC, so east of Greenwich the
+// window was shifted by the offset and part of the evening fell outside
+// "today": on a +04 box after 20:00 UTC the poller saw an empty calendar. The
+// owner's day is the day the day plan and the busy gate mean.
+func ParseICalDay(body []byte, now time.Time) []Event {
+	y, m, d := now.Date()
+	start := time.Date(y, m, d, 0, 0, 0, 0, now.Location())
+	return ParseICal(body, start, start.AddDate(0, 0, 1))
+}
+
+// parseVEVENT extracts UID, start, end and summary from a VEVENT block.
+// Reports false for all-day events and parse failures.
+func parseVEVENT(block string) (Event, bool) {
+	var e Event
+	for _, line := range strings.Split(block, "\n") {
+		line = strings.TrimSpace(line)
+		switch {
+		case strings.HasPrefix(line, "DTSTART"):
+			if t, ok := parseDT(line); ok {
+				e.Start = t
+			}
+		case strings.HasPrefix(line, "DTEND"):
+			if t, ok := parseDT(line); ok {
+				e.End = t
+			}
+		case strings.HasPrefix(line, "SUMMARY"):
+			e.Summary = afterColon(line)
+		case strings.HasPrefix(line, "UID"):
+			e.UID = afterColon(line)
+		}
+	}
+	if e.Start.IsZero() || e.End.IsZero() {
+		return Event{}, false
+	}
+	return e, true
+}
+
+func afterColon(line string) string {
+	if i := strings.Index(line, ":"); i >= 0 {
+		return strings.TrimSpace(line[i+1:])
+	}
+	return ""
+}
+
+// parseDT parses a DTSTART/DTEND value:
+//
+//   - UTC:      DTEND:20260703T100000Z
+//   - Local:    DTSTART;TZID=Europe/Moscow:20260703T130000
+//   - All-day:  DTSTART;VALUE=DATE:20260703  (rejected)
+//
+// A local time is read as UTC, the behaviour cmd/mavcaldav has always had: the
+// CalDAV server and the poller run in the same timezone, and the busy gate only
+// needs busy/not-busy to be right.
+func parseDT(line string) (time.Time, bool) {
+	if strings.Contains(line, "VALUE=DATE:") {
+		return time.Time{}, false
+	}
+	i := strings.LastIndex(line, ":")
+	if i < 0 {
+		return time.Time{}, false
+	}
+	val := strings.TrimSuffix(strings.TrimSpace(line[i+1:]), "Z")
+	t, err := time.Parse("20060102T150405", val)
+	if err != nil {
+		return time.Time{}, false
+	}
+	return t.UTC(), true
+}
+
+// RenderICal wraps events in a VCALENDAR body suitable for PUTting to a CalDAV
+// collection. One event per file is the CalDAV convention, so callers normally
+// pass a single event.
+//
+// This is the write half of #127 and it only ever renders: the canonical state
+// is sqlite, the calendar is a view of it. Nothing reads a rendered file back.
+func RenderICal(events []Event) string {
+	var b strings.Builder
+	b.WriteString("BEGIN:VCALENDAR\r\nVERSION:2.0\r\nPRODID:-//maven//local calendar//RU\r\n")
+	for _, e := range events {
+		b.WriteString("BEGIN:VEVENT\r\n")
+		fmt.Fprintf(&b, "UID:%s\r\n", escapeText(e.UID))
+		fmt.Fprintf(&b, "DTSTAMP:%s\r\n", e.Start.UTC().Format("20060102T150405Z"))
+		fmt.Fprintf(&b, "DTSTART:%s\r\n", e.Start.UTC().Format("20060102T150405Z"))
+		fmt.Fprintf(&b, "DTEND:%s\r\n", e.End.UTC().Format("20060102T150405Z"))
+		fmt.Fprintf(&b, "SUMMARY:%s\r\n", escapeText(e.Summary))
+		b.WriteString("END:VEVENT\r\n")
+	}
+	b.WriteString("END:VCALENDAR\r\n")
+	return b.String()
+}
+
+// escapeText applies RFC 5545 TEXT escaping and strips the line breaks that
+// would otherwise let a reminder payload inject iCal properties.
+func escapeText(s string) string {
+	s = strings.ReplaceAll(s, "\\", "\\\\")
+	s = strings.ReplaceAll(s, ";", "\\;")
+	s = strings.ReplaceAll(s, ",", "\\,")
+	s = strings.ReplaceAll(s, "\r\n", "\\n")
+	s = strings.ReplaceAll(s, "\n", "\\n")
+	s = strings.ReplaceAll(s, "\r", "\\n")
+	return s
+}

internal/calendar/ical_render_test.go

@@ -0,0 +1,69 @@
+package calendar
+
+import (
+	"strings"
+	"testing"
+	"time"
+)
+
+func TestRenderICalRoundTrips(t *testing.T) {
+	e := ReminderEvent(7, time.Date(2026, 8, 1, 18, 30, 0, 0, time.UTC), "позвонить маме", 0)
+	if e.UID != "maven-reminder-7" {
+		t.Errorf("UID = %q", e.UID)
+	}
+	if got := e.End.Sub(e.Start); got != DefaultReminderDuration {
+		t.Errorf("duration = %v, want %v", got, DefaultReminderDuration)
+	}
+	if got, want := ReminderPath(7), "maven-reminder-7.ics"; got != want {
+		t.Errorf("ReminderPath = %q, want %q", got, want)
+	}
+
+	body := RenderICal([]Event{e})
+	if !strings.HasPrefix(body, "BEGIN:VCALENDAR\r\n") || !strings.HasSuffix(body, "END:VCALENDAR\r\n") {
+		t.Fatalf("not a VCALENDAR body:\n%s", body)
+	}
+
+	back := ParseICal([]byte(body), e.Start.Add(-time.Hour), e.Start.Add(time.Hour))
+	if len(back) != 1 {
+		t.Fatalf("got %d events back, want 1:\n%s", len(back), body)
+	}
+	if back[0].UID != e.UID || back[0].Summary != e.Summary {
+		t.Errorf("round trip lost identity: %+v", back[0])
+	}
+	if !back[0].Start.Equal(e.Start) || !back[0].End.Equal(e.End) {
+		t.Errorf("round trip lost times: %+v", back[0])
+	}
+}
+
+func TestRenderICalIsDeterministic(t *testing.T) {
+	e := ReminderEvent(1, time.Date(2026, 8, 1, 9, 0, 0, 0, time.UTC), "выпить воды", 0)
+	if RenderICal([]Event{e}) != RenderICal([]Event{e}) {
+		t.Error("the same reminder must render byte-identically, or every poll re-PUTs it")
+	}
+}
+
+// A reminder payload is owner-supplied text. It must not be able to close the
+// VEVENT and inject properties of its own.
+func TestRenderICalEscapesInjection(t *testing.T) {
+	e := ReminderEvent(2, time.Date(2026, 8, 1, 9, 0, 0, 0, time.UTC),
+		"обед\r\nEND:VEVENT\r\nBEGIN:VEVENT\r\nSUMMARY:injected", 0)
+	body := RenderICal([]Event{e})
+	// Count line-initial occurrences: the escaped text still contains the
+	// characters "BEGIN:VEVENT", it just can no longer start a line.
+	if n := strings.Count(body, "\r\nBEGIN:VEVENT\r\n"); n != 1 {
+		t.Fatalf("payload injected a second VEVENT (%d):\n%s", n, body)
+	}
+	if n := strings.Count(body, "\r\nEND:VEVENT\r\n"); n != 1 {
+		t.Fatalf("payload closed the VEVENT early (%d):\n%s", n, body)
+	}
+	if !strings.Contains(body, `SUMMARY:обед\nEND:VEVENT`) {
+		t.Errorf("newlines should be escaped, not dropped:\n%s", body)
+	}
+}
+
+func TestReminderEventEmptyPayload(t *testing.T) {
+	e := ReminderEvent(3, time.Date(2026, 8, 1, 9, 0, 0, 0, time.UTC), "   ", 0)
+	if e.Summary != "напоминание" {
+		t.Errorf("Summary = %q, want the neutral RU fallback", e.Summary)
+	}
+}

internal/calendar/reminder.go

@@ -0,0 +1,45 @@
+package calendar
+
+import (
+	"fmt"
+	"strings"
+	"time"
+)
+
+// ReminderUIDPrefix namespaces every event maven publishes. Two reasons it is a
+// fixed prefix and not a random UUID: the render is idempotent (the same
+// reminder always lands on the same UID, so re-rendering overwrites instead of
+// duplicating), and everything maven owns in the target collection is
+// identifiable at a glance — she never touches a file she did not create.
+const ReminderUIDPrefix = "maven-reminder-"
+
+// DefaultReminderDuration — how long a rendered reminder occupies. A reminder
+// is an instant, a calendar entry is a span, so one has to be invented; 30
+// minutes reads as a block in a calendar app without swallowing the afternoon.
+const DefaultReminderDuration = 30 * time.Minute
+
+// ReminderEvent maps a reminder to the event that represents it. id and fire
+// come from the store; payload is the RU text as the owner said it, rendered
+// verbatim as the summary — the calendar is a view of sqlite, not a place to
+// rephrase.
+func ReminderEvent(id int64, fire time.Time, payload string, dur time.Duration) Event {
+	if dur <= 0 {
+		dur = DefaultReminderDuration
+	}
+	summary := strings.TrimSpace(payload)
+	if summary == "" {
+		summary = "напоминание"
+	}
+	return Event{
+		UID:     fmt.Sprintf("%s%d", ReminderUIDPrefix, id),
+		Summary: summary,
+		Start:   fire,
+		End:     fire.Add(dur),
+	}
+}
+
+// ReminderPath is the collection-relative filename for a rendered reminder.
+// One event per resource, per the CalDAV convention.
+func ReminderPath(id int64) string {
+	return fmt.Sprintf("%s%d.ics", ReminderUIDPrefix, id)
+}