init

utils/formatBriefTimestamp.ts

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+/**
+ * Format an ISO timestamp for the brief/chat message label line.
+ *
+ * Display scales with age (like a messaging app):
+ *   - same day:      "1:30 PM" or "13:30" (locale-dependent)
+ *   - within 6 days: "Sunday, 4:15 PM" (locale-dependent)
+ *   - older:         "Sunday, Feb 20, 4:30 PM" (locale-dependent)
+ *
+ * Respects POSIX locale env vars (LC_ALL > LC_TIME > LANG) for time format
+ * (12h/24h), weekday names, month names, and overall structure.
+ * Bun/V8's `toLocaleString(undefined)` ignores these on macOS, so we
+ * convert them to BCP 47 tags ourselves.
+ *
+ * `now` is injectable for tests.
+ */
+export function formatBriefTimestamp(
+  isoString: string,
+  now: Date = new Date(),
+): string {
+  const d = new Date(isoString)
+  if (Number.isNaN(d.getTime())) {
+    return ''
+  }
+
+  const locale = getLocale()
+  const dayDiff = startOfDay(now) - startOfDay(d)
+  const daysAgo = Math.round(dayDiff / 86_400_000)
+
+  if (daysAgo === 0) {
+    return d.toLocaleTimeString(locale, {
+      hour: 'numeric',
+      minute: '2-digit',
+    })
+  }
+
+  if (daysAgo > 0 && daysAgo < 7) {
+    return d.toLocaleString(locale, {
+      weekday: 'long',
+      hour: 'numeric',
+      minute: '2-digit',
+    })
+  }
+
+  return d.toLocaleString(locale, {
+    weekday: 'long',
+    month: 'short',
+    day: 'numeric',
+    hour: 'numeric',
+    minute: '2-digit',
+  })
+}
+
+/**
+ * Derive a BCP 47 locale tag from POSIX env vars.
+ * LC_ALL > LC_TIME > LANG, falls back to undefined (system default).
+ * Converts POSIX format (en_GB.UTF-8) to BCP 47 (en-GB).
+ */
+function getLocale(): string | undefined {
+  const raw =
+    process.env.LC_ALL || process.env.LC_TIME || process.env.LANG || ''
+  if (!raw || raw === 'C' || raw === 'POSIX') {
+    return undefined
+  }
+  // Strip codeset (.UTF-8) and modifier (@euro), replace _ with -
+  const base = raw.split('.')[0]!.split('@')[0]!
+  if (!base) {
+    return undefined
+  }
+  const tag = base.replaceAll('_', '-')
+  // Validate by trying to construct an Intl locale — invalid tags throw
+  try {
+    new Intl.DateTimeFormat(tag)
+    return tag
+  } catch {
+    return undefined
+  }
+}
+
+function startOfDay(d: Date): number {
+  return new Date(d.getFullYear(), d.getMonth(), d.getDate()).getTime()
+}