notifications: timezone fix + configurable site timezone

frontend/src/utils/datetime.js

@@ -0,0 +1,80 @@
+// Site-timezone datetime helpers.
+//
+// Notification start/end times are stored as UTC and must be shown + entered in
+// the site's configured IANA timezone (settings key site_timezone), not the
+// viewer's browser zone, so every site and remote admin agrees on the wall
+// clock. These convert between a UTC instant and a 'YYYY-MM-DDTHH:mm' string in
+// a given zone (the value a datetime-local input holds).
+
+const DEFAULT_TZ = 'America/New_York'
+
+function pad(n) {
+  return String(n).padStart(2, '0')
+}
+
+// Wall-clock parts of a UTC instant AS SEEN in tz. Uses Intl so it is correct
+// across DST without shipping a tz table.
+function zonedParts(date, tz) {
+  const dtf = new Intl.DateTimeFormat('en-US', {
+    timeZone: tz,
+    year: 'numeric', month: '2-digit', day: '2-digit',
+    hour: '2-digit', minute: '2-digit', second: '2-digit',
+    hourCycle: 'h23',
+  })
+  const map = {}
+  for (const p of dtf.formatToParts(date)) {
+    if (p.type !== 'literal') map[p.type] = p.value
+  }
+  return {
+    year: Number(map.year), month: Number(map.month), day: Number(map.day),
+    hour: Number(map.hour), minute: Number(map.minute), second: Number(map.second),
+  }
+}
+
+// tz offset (ms) at the given instant: (wall-clock-as-UTC) - (real UTC).
+function tzOffsetMs(date, tz) {
+  const p = zonedParts(date, tz)
+  const asUtc = Date.UTC(p.year, p.month - 1, p.day, p.hour, p.minute, p.second)
+  return asUtc - date.getTime()
+}
+
+// UTC instant (or ISO string) -> 'YYYY-MM-DDTHH:mm' wall clock in tz, for a
+// datetime-local input. Empty string on no/invalid input.
+export function zonedInputFromUtc(value, tz = DEFAULT_TZ) {
+  if (!value) return ''
+  const date = value instanceof Date ? value : new Date(value)
+  if (isNaN(date.getTime())) return ''
+  const p = zonedParts(date, tz)
+  return `${p.year}-${pad(p.month)}-${pad(p.day)}T${pad(p.hour)}:${pad(p.minute)}`
+}
+
+// 'YYYY-MM-DDTHH:mm' wall clock in tz -> UTC ISO string. Interprets the entered
+// time as tz-local (DST-aware) and returns the real instant. Empty -> null.
+export function utcFromZonedInput(localStr, tz = DEFAULT_TZ) {
+  if (!localStr) return null
+  const [datePart, timePart = '00:00'] = localStr.split('T')
+  const [y, mo, d] = datePart.split('-').map(Number)
+  const [h, mi] = timePart.split(':').map(Number)
+  if ([y, mo, d, h, mi].some(Number.isNaN)) return null
+  // Guess the instant as if the wall clock were UTC, then subtract the zone
+  // offset AT that instant. One correction is exact except in the ~1h/yr DST
+  // fold, where either side is a fair reading.
+  const guess = Date.UTC(y, mo - 1, d, h, mi)
+  const offset = tzOffsetMs(new Date(guess), tz)
+  return new Date(guess - offset).toISOString()
+}
+
+// UTC instant (or ISO) -> human string in tz, e.g. "Jul 30, 2026, 2:34 PM".
+export function formatInZone(value, tz = DEFAULT_TZ, opts = {}) {
+  if (!value) return ''
+  const date = value instanceof Date ? value : new Date(value)
+  if (isNaN(date.getTime())) return ''
+  return new Intl.DateTimeFormat('en-US', {
+    timeZone: tz,
+    year: 'numeric', month: 'short', day: 'numeric',
+    hour: 'numeric', minute: '2-digit',
+    ...opts,
+  }).format(date)
+}
+
+export { DEFAULT_TZ }