Allow backdating a routine completion

1 files changed, 26 insertions, 0 deletions

diff --git a/src/lib/shared/temporal.ts b/src/lib/shared/temporal.ts
index 103f209..6fd085d 100644
--- a/src/lib/shared/temporal.ts
+++ b/src/lib/shared/temporal.ts
@@ -24,3 +24,29 @@ export function formatDayMonthYear(iso: string): string {
 		year: 'numeric',
 	});
 }
+
+const ONE_DAY_MS = 86_400_000;
+
+/**
+ * Stamp to store for a `YYYY-MM-DD` form day: noon UTC — a midnight stamp can
+ * read back as the wrong day in a timezone ahead of UTC. Clamped to `now` when
+ * the day is still ahead of the clock, and null for a malformed day or one
+ * more than a day in the future (timezones ahead of UTC make "today" fuzzy).
+ */
+export function instantFromDay(
+	day: string,
+	now: Temporal.Instant,
+): string | null {
+	let midnight: Temporal.Instant;
+	try {
+		midnight = Temporal.PlainDate.from(day.trim())
+			.toZonedDateTime('UTC')
+			.toInstant();
+	} catch {
+		return null;
+	}
+	const noon = midnight.add({ hours: 12 });
+	if (noon.epochMilliseconds - now.epochMilliseconds > ONE_DAY_MS) return null;
+	const stamp = Temporal.Instant.compare(noon, now) > 0 ? now : noon;
+	return stamp.toString({ smallestUnit: 'millisecond' });
+}