Make UI language, timestamps and live feedback consistent

src/lib/time-zone.ts

@@ -17,7 +17,7 @@ export function availableTimeZones(): string[] {
     return [...new Set([DEFAULT_TIME_ZONE, 'UTC', ...zones])].sort();
 }
 
-type Timestamp = string | number | Date | null | undefined;
+export type Timestamp = string | number | Date | null | undefined;
 
 /** Database timestamps without an offset are UTC, never browser-local time. */
 function parseTimestamp(value: Exclude<Timestamp, null | undefined>): Date {
@@ -28,6 +28,12 @@ function parseTimestamp(value: Exclude<Timestamp, null | undefined>): Date {
     return new Date(value);
 }
 
+export function timestampISO(value: Timestamp): string | undefined {
+    if (value === null || value === undefined || value === '') return undefined;
+    const parsed = parseTimestamp(value);
+    return Number.isFinite(parsed.getTime()) ? parsed.toISOString() : undefined;
+}
+
 export function createTimeFormatter(zone: string) {
     const timeZone = validTimeZone(zone) ? zone : DEFAULT_TIME_ZONE;
     const date: Intl.DateTimeFormatOptions = { year: 'numeric', month: 'short', day: 'numeric' };
@@ -40,12 +46,34 @@ export function createTimeFormatter(zone: string) {
             return Number.isFinite(parsed.getTime()) ? format.format(parsed) : '—';
         };
     }
+    const absolute = formatter({ ...date, hour: 'numeric', minute: '2-digit', timeZoneName: 'short' });
+    const clock = formatter({ hour: 'numeric', minute: '2-digit' });
+    const calendar = new Intl.DateTimeFormat('en-US', { timeZone, year: 'numeric', month: 'numeric', day: 'numeric' });
+    const calendarDay = (value: Date) => {
+        const parts = Object.fromEntries(calendar.formatToParts(value).map(part => [part.type, part.value]));
+        return Date.UTC(Number(parts.year), Number(parts.month) - 1, Number(parts.day));
+    };
+    function relative(value: Timestamp, now = Date.now()): string {
+        if (value === null || value === undefined || value === '') return '—';
+        const parsed = parseTimestamp(value);
+        if (!Number.isFinite(parsed.getTime()) || !Number.isFinite(now)) return '—';
+        const age = now - parsed.getTime();
+        if (age < 0) return absolute(parsed);
+        if (age < 5_000) return 'Just now';
+        if (age < 60_000) return `${Math.floor(age / 1000)} s ago`;
+        if (age < 3_600_000) return `${Math.floor(age / 60_000)} min ago`;
+        if (age < 86_400_000) return `${Math.floor(age / 3_600_000)} h ago`;
+        // Calendar arithmetic keeps Yesterday correct across DST and year changes.
+        if (calendarDay(new Date(now)) - calendarDay(parsed) === 86_400_000) return `Yesterday, ${clock(parsed)}`;
+        return absolute(parsed);
+    }
     return {
+        relative,
         dateTime: formatter({ ...date, ...time }),
         date: formatter(date),
         time: formatter(time),
         clockTime: formatter({ hour: 'numeric', minute: '2-digit', second: '2-digit' }),
         minuteTime: formatter({ hour: 'numeric', minute: '2-digit', timeZoneName: 'short' }),
-        period: formatter({ ...date, hour: 'numeric', minute: '2-digit', timeZoneName: 'short' })
+        period: absolute
     };
 }