calendar: read a wall clock as a wall clock, and unfold iCal (V-581)

FactSpan built both instants by adding a duration to local midnight, so on
the two DST changeover days every span was an hour off. A day is 23 or 25
hours wide there, and the busy gate then read a 14:00 meeting as 13:00 or
15:00. Both readings are time.Date now, and the midnight crossing is AddDate
rather than a 24-hour add.

The iCal parse did not unfold content lines. A server folds a property at 75
octets and a Russian summary is two bytes a letter, so the tail of an
ordinary weekly standup was read as an unknown property and dropped, and the
event was filed under a truncated name. RFC 5545 TEXT escapes are also
reversed now, which RenderICal has always written and the parse never undid.

Two regression tests: a folded and escaped summary, and a span across the
start of DST in Europe/Berlin.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

internal/calendar/calendar.go

@@ -140,6 +140,12 @@ const EventKeyPrefix = "calendar_event_"
 //
 // An end at or before the start is read as crossing midnight, so a 23:30-00:15
 // meeting covers the quarter hour it actually covers.
+//
+// Both readings are built with time.Date rather than added to midnight as a
+// duration. A day is 23 or 25 hours wide on the two DST changeovers, so
+// midnight plus fourteen hours is 13:00 or 15:00 on those days, and the busy
+// gate would then read a 14:00 meeting an hour off. The same goes for the
+// midnight crossing, which is AddDate and not a 24-hour add.
 func FactSpan(key, value string, loc *time.Location) (start, end time.Time, ok bool) {
 	if !strings.HasPrefix(key, EventKeyPrefix) {
 		return time.Time{}, time.Time{}, false
@@ -172,10 +178,11 @@ func FactSpan(key, value string, loc *time.Location) (start, end time.Time, ok b
 	if !ok1 || !ok2 {
 		return time.Time{}, time.Time{}, false
 	}
-	start = day.Add(time.Duration(sh)*time.Hour + time.Duration(sm)*time.Minute)
-	end = day.Add(time.Duration(eh)*time.Hour + time.Duration(em)*time.Minute)
+	y, mo, d := day.Date()
+	start = time.Date(y, mo, d, sh, sm, 0, 0, loc)
+	end = time.Date(y, mo, d, eh, em, 0, 0, loc)
 	if !end.After(start) {
-		end = end.Add(24 * time.Hour)
+		end = end.AddDate(0, 0, 1)
 	}
 	return start, end, true
 }

internal/calendar/ical.go

@@ -66,7 +66,7 @@ func ParseICalDay(body []byte, now time.Time) []Event {
 // Reports false for all-day events and parse failures.
 func parseVEVENT(block string, loc *time.Location) (Event, bool) {
 	var e Event
-	for _, line := range strings.Split(block, "\n") {
+	for _, line := range strings.Split(unfold(block), "\n") {
 		line = strings.TrimSpace(line)
 		switch {
 		case strings.HasPrefix(line, "DTSTART"):
@@ -78,9 +78,9 @@ func parseVEVENT(block string, loc *time.Location) (Event, bool) {
 				e.End = t
 			}
 		case strings.HasPrefix(line, "SUMMARY"):
-			e.Summary = afterColon(line)
+			e.Summary = unescapeText(afterColon(line))
 		case strings.HasPrefix(line, "UID"):
-			e.UID = afterColon(line)
+			e.UID = unescapeText(afterColon(line))
 		}
 	}
 	if e.Start.IsZero() || e.End.IsZero() {
@@ -89,6 +89,48 @@ func parseVEVENT(block string, loc *time.Location) (Event, bool) {
 	return e, true
 }
 
+// unfold undoes RFC 5545 content-line folding, where a long property is split
+// with a CRLF and the continuation begins with one space or tab.
+//
+// It runs before the block is split into lines, because splitting first and
+// trimming each line destroys the leading space that marks a continuation. A
+// server folds at 75 octets and a Russian summary is two bytes a letter, so
+// "Еженедельная планёрка с командой" crosses the limit easily — without this
+// the tail of the summary was read as an unknown property and dropped, and the
+// event was filed under a truncated name.
+func unfold(block string) string {
+	if !strings.Contains(block, "\n ") && !strings.Contains(block, "\n\t") {
+		return block
+	}
+	return strings.NewReplacer("\r\n ", "", "\r\n\t", "", "\n ", "", "\n\t", "").Replace(block)
+}
+
+// unescapeText reverses the RFC 5545 TEXT escaping escapeText applies. Without
+// it a summary a server wrote as "Обед\, потом созвон" reaches the day plan
+// with the backslash still in it, and FactKey folds that literal into the key.
+func unescapeText(s string) string {
+	if !strings.Contains(s, `\`) {
+		return s
+	}
+	var b strings.Builder
+	b.Grow(len(s))
+	for i := 0; i < len(s); i++ {
+		if s[i] != '\\' || i+1 >= len(s) {
+			b.WriteByte(s[i])
+			continue
+		}
+		i++
+		switch s[i] {
+		case 'n', 'N':
+			b.WriteByte('\n')
+		default:
+			// ";", ",", "\\" and anything else a writer escaped needlessly.
+			b.WriteByte(s[i])
+		}
+	}
+	return b.String()
+}
+
 func afterColon(line string) string {
 	if i := strings.Index(line, ":"); i >= 0 {
 		return strings.TrimSpace(line[i+1:])

internal/calendar/ical_render_test.go

@@ -61,6 +61,44 @@ func TestRenderICalEscapesInjection(t *testing.T) {
 	}
 }
 
+// A folded SUMMARY is one property, not a property plus a dropped tail. Servers
+// fold at 75 octets and a Russian summary is two bytes a letter.
+func TestParseICalUnfoldsAndUnescapes(t *testing.T) {
+	body := []byte("BEGIN:VEVENT\r\n" +
+		"UID:u1\r\n" +
+		"DTSTART:20260703T130000Z\r\n" +
+		"DTEND:20260703T140000Z\r\n" +
+		"SUMMARY:Еженедельная планёрка\\, потом\r\n  созвон\r\n" +
+		"END:VEVENT\r\n")
+	from := time.Date(2026, 7, 3, 0, 0, 0, 0, time.UTC)
+	events := ParseICal(body, from, from.AddDate(0, 0, 1))
+	if len(events) != 1 {
+		t.Fatalf("got %d events, want 1", len(events))
+	}
+	if want := "Еженедельная планёрка, потом созвон"; events[0].Summary != want {
+		t.Errorf("Summary = %q, want %q", events[0].Summary, want)
+	}
+}
+
+// A day is 23 hours wide where DST starts, so a wall clock reading has to be
+// built with time.Date and never as midnight plus a duration.
+func TestFactSpanAcrossDSTStart(t *testing.T) {
+	loc, err := time.LoadLocation("Europe/Berlin")
+	if err != nil {
+		t.Skipf("no tzdata for Europe/Berlin: %v", err)
+	}
+	start, end, ok := FactSpan("calendar_event_20260329_Planerka", "Planerka @ 14:00-15:00", loc)
+	if !ok {
+		t.Fatal("FactSpan reported not ok")
+	}
+	if start.Hour() != 14 || start.Minute() != 0 {
+		t.Errorf("start = %s, want a 14:00 wall clock", start)
+	}
+	if end.Hour() != 15 {
+		t.Errorf("end = %s, want a 15:00 wall clock", end)
+	}
+}
+
 func TestReminderEventEmptyPayload(t *testing.T) {
 	e := ReminderEvent(3, time.Date(2026, 8, 1, 9, 0, 0, 0, time.UTC), "   ", 0)
 	if e.Summary != "напоминание" {