Review fixes: the database restore also reads a damaged blob once more; schedule conversion right on days the clocks change; the field follows a changed schedule time zone

src/format.js

@@ -33,17 +33,23 @@ export function formatSize(bytes) {
  * @param {string} hhmm the time
  * @param {string} from IANA zone it is in
  * @param {string} to IANA zone to show it in
+ * @param {Date} now the day to convert on (today)
  * @return {string}
  */
-export function toZone(hhmm, from, to) {
+export function toZone(hhmm, from, to, now = new Date()) {
 	const [h, m] = hhmm.split(':').map(Number)
 	const parts = (zone, date) => Object.fromEntries(new Intl.DateTimeFormat('en-GB', {
 		timeZone: zone, hourCycle: 'h23', year: 'numeric', month: '2-digit', day: '2-digit', hour: '2-digit', minute: '2-digit',
 	}).formatToParts(date).map((p) => [p.type, p.value]))
-	const today = parts(from, new Date())
-	const guess = Date.UTC(+today.year, +today.month - 1, +today.day, h, m)
-	const seen = parts(from, new Date(guess))
-	const offset = Date.UTC(+seen.year, +seen.month - 1, +seen.day, +seen.hour, +seen.minute) - guess
-	const there = parts(to, new Date(guess - offset))
+	const offsetAt = (at) => {
+		const seen = parts(from, new Date(at))
+		return Date.UTC(+seen.year, +seen.month - 1, +seen.day, +seen.hour, +seen.minute) - at
+	}
+	const today = parts(from, now)
+	const wall = Date.UTC(+today.year, +today.month - 1, +today.day, h, m)
+	// the offset at the wall time itself: on a day the clocks change it differs from the one at
+	// midnight, so it is looked up a second time at the moment found first
+	const at = wall - offsetAt(wall - offsetAt(wall))
+	const there = parts(to, new Date(at))
 	return `${there.hour}:${there.minute}`
 }