fix(ics): a date-only task now shows in a subscribed calendar

The calendar feed emitted every task as a VTODO and, on top of that, an event only
for tasks that carried a clock time. But a subscribed calendar in Apple Calendar or
Google ignores VTODO entirely and draws only events — so a task with just a due
date, which is most of them, appeared in the verdande app's own calendar and
nowhere in the calendar it had been subscribed into. "Verdande does not update in my
calendar, only in the verdande app."

Every dated, unfinished task is an event now: a clock time makes a timed event as
before, a bare date an all-day one (a DATE start and an exclusive next-day end, so
it stays on its own day). Finished tasks stay off the calendar, as they were.

Test updated: the sample's timed task and its two date-only tasks all become events
now, the all-day ones in the DATE form, and the finished one still makes none.

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

internal/ics/ics.go

@@ -62,8 +62,13 @@ func Render(cal Calendar) string {
 
 	for _, t := range cal.Tasks {
 		writeTodo(&b, t, cal.Domain)
-		// A dated task also goes out as an event, for clients that ignore VTODO.
-		if t.DueDatetime != nil && !t.Completed {
+		// A dated task also goes out as an event, for the calendar apps that ignore
+		// VTODO — which is all of them: Apple Calendar and Google both drop a
+		// subscribed to-do on the floor and show only events. A task with a clock
+		// time becomes a timed event; one with only a date, an all-day event. Once
+		// this was limited to timed tasks, so a date-only task — most of them —
+		// simply never appeared in the calendar it was subscribed into.
+		if !t.Completed && (t.DueDatetime != nil || t.DueDate != "") {
 			writeEvent(&b, t, cal.Domain)
 		}
 	}
@@ -128,22 +133,31 @@ func writeTodo(b *strings.Builder, t Task, domain string) {
 }
 
 func writeEvent(b *strings.Builder, t Task, domain string) {
-	start := *t.DueDatetime
-	// Without a stated duration a task is shown as half an hour. A zero-length
-	// event collapses to an invisible sliver in most calendar grids.
-	minutes := 30
-	if t.DurationMin != nil && *t.DurationMin > 0 {
-		minutes = *t.DurationMin
-	}
-	end := start.Add(time.Duration(minutes) * time.Minute)
-
 	b.WriteString("BEGIN:VEVENT\r\n")
 	// A distinct UID suffix: the event and the to-do describe the same task, and
 	// sharing a UID would make a client treat them as two versions of one item.
 	writeLine(b, "UID:"+t.ID+"-event@"+domain)
 	writeLine(b, "DTSTAMP:"+utc(t.UpdatedAt))
-	writeLine(b, "DTSTART:"+utc(start))
-	writeLine(b, "DTEND:"+utc(end))
+
+	if t.DueDatetime != nil {
+		start := *t.DueDatetime
+		// Without a stated duration a task is shown as half an hour. A zero-length
+		// event collapses to an invisible sliver in most calendar grids.
+		minutes := 30
+		if t.DurationMin != nil && *t.DurationMin > 0 {
+			minutes = *t.DurationMin
+		}
+		writeLine(b, "DTSTART:"+utc(start))
+		writeLine(b, "DTEND:"+utc(start.Add(time.Duration(minutes)*time.Minute)))
+	} else {
+		// A date with no time is an all-day event. DTEND is exclusive — a one-day
+		// event ends on the following date — which is what keeps it a single day in
+		// the grid rather than bleeding into the next.
+		day := strings.ReplaceAll(t.DueDate, "-", "")
+		writeLine(b, "DTSTART;VALUE=DATE:"+day)
+		writeLine(b, "DTEND;VALUE=DATE:"+nextDay(t.DueDate))
+	}
+
 	writeLine(b, "SUMMARY:"+escape(t.Content))
 	if t.Description != "" {
 		writeLine(b, "DESCRIPTION:"+escape(t.Description))
@@ -157,6 +171,17 @@ func writeEvent(b *strings.Builder, t Task, domain string) {
 
 func utc(t time.Time) string { return t.UTC().Format("20060102T150405Z") }
 
+// nextDay is the YYYYMMDD after a YYYY-MM-DD date, for an all-day event's exclusive
+// DTEND. A malformed date is returned as its own compacted form rather than
+// dropped, so a task never loses its event over a bad string.
+func nextDay(date string) string {
+	d, err := time.Parse("2006-01-02", date)
+	if err != nil {
+		return strings.ReplaceAll(date, "-", "")
+	}
+	return d.AddDate(0, 0, 1).Format("20060102")
+}
+
 // escape applies RFC 5545 text escaping. A comma or semicolon in a task title is
 // a value separator in iCalendar, so an unescaped one silently truncates the
 // summary at that character in every client.