Migrate date handling from Date to Temporal API

Routine scheduling, health series, service stamps, session expiry, and
date formatting move to native Temporal (Instant / ZonedDateTime in
UTC), with ISO strings kept at DB and chart boundaries. Ambient types
in src/temporal-ambient.d.ts cover the native runtime; no polyfill.
Observable behavior and stored formats are unchanged.

1 files changed, 27 insertions, 0 deletions

diff --git a/src/lib/shared/temporal.ts b/src/lib/shared/temporal.ts
new file mode 100644
index 0000000..7097ba3
--- /dev/null
+++ b/src/lib/shared/temporal.ts
@@ -0,0 +1,27 @@
+// Shared Temporal display helpers. Storage and logic stay in ISO instants;
+// these format one for the viewer in their own timezone.
+
+/** Format an ISO instant in the viewer's timezone. */
+export function formatInstant(
+	iso: string,
+	options: Intl.DateTimeFormatOptions,
+): string {
+	return Temporal.Instant.from(iso)
+		.toZonedDateTimeISO(Temporal.Now.timeZoneId())
+		.toLocaleString('fr-FR', options);
+}
+
+/** Short day-month stamp ("8 sept."). */
+
+export function formatDayMonth(iso: string): string {
+	return formatInstant(iso, { day: 'numeric', month: 'short' });
+}
+
+/** Day-month-year stamp ("8 sept. 2026"). */
+export function formatDayMonthYear(iso: string): string {
+	return formatInstant(iso, {
+		day: 'numeric',
+		month: 'short',
+		year: 'numeric',
+	});
+}