Convert between epoch (s or ms), ISO 8601, and any IANA timezone. Give a value (epoch seconds or milliseconds, an ISO string, or 'now'), get every representation back: utc, epochSeconds, epochMillis and, with tz, timezone, localIso (ISO 8601 with the zone's offset, e.g. 2026-06-11T19:00:00+09:00), utcOffset and local (a display string). A bare number under 1e12 is read as seconds, otherwise as milliseconds.