Add timezone preference across time displays

src/lib/time-zone.ts

@@ -0,0 +1,50 @@
+export const DEFAULT_TIME_ZONE = 'America/New_York';
+
+export function validTimeZone(value: unknown): value is string {
+    if (typeof value !== 'string' || !value.trim()) return false;
+    try {
+        new Intl.DateTimeFormat('en-US', { timeZone: value });
+        return true;
+    } catch { return false; }
+}
+
+export function availableTimeZones(): string[] {
+    // Older browsers can still select common zones when enumeration is unavailable.
+    const fallback = [DEFAULT_TIME_ZONE, 'America/Chicago', 'America/Denver', 'America/Los_Angeles',
+        'America/Anchorage', 'Pacific/Honolulu', 'Europe/London', 'Europe/Paris',
+        'Asia/Kolkata', 'Asia/Tokyo', 'Australia/Sydney'];
+    const zones = typeof Intl.supportedValuesOf === 'function' ? Intl.supportedValuesOf('timeZone') : fallback;
+    return [...new Set([DEFAULT_TIME_ZONE, 'UTC', ...zones])].sort();
+}
+
+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 {
+    if (value instanceof Date) return value;
+    if (typeof value === 'string' && /^\d{4}-\d{2}-\d{2}[ T]\d{2}:\d{2}:\d{2}(\.\d+)?$/.test(value)) {
+        return new Date(value.replace(' ', 'T') + 'Z');
+    }
+    return new Date(value);
+}
+
+export function createTimeFormatter(zone: string) {
+    const timeZone = validTimeZone(zone) ? zone : DEFAULT_TIME_ZONE;
+    const date: Intl.DateTimeFormatOptions = { year: 'numeric', month: 'short', day: 'numeric' };
+    const time: Intl.DateTimeFormatOptions = { hour: 'numeric', minute: '2-digit', second: '2-digit', timeZoneName: 'short' };
+    function formatter(options: Intl.DateTimeFormatOptions) {
+        const format = new Intl.DateTimeFormat('en-US', { ...options, timeZone });
+        return (value: Timestamp): string => {
+            if (value === null || value === undefined || value === '') return '—';
+            const parsed = parseTimestamp(value);
+            return Number.isFinite(parsed.getTime()) ? format.format(parsed) : '—';
+        };
+    }
+    return {
+        dateTime: formatter({ ...date, ...time }),
+        date: formatter(date),
+        time: formatter(time),
+        minuteTime: formatter({ hour: 'numeric', minute: '2-digit', timeZoneName: 'short' }),
+        period: formatter({ ...date, hour: 'numeric', minute: '2-digit', timeZoneName: 'short' })
+    };
+}