Accurate Time Clock

Displays the current time across major world cities and time zones in real time. Updates every second based on the user's system clock, then offsets each zone using the browser's IANA tz data.

Includes a digital and analog clock view, with optional 24-hour formatting and seconds display.

The display is computed entirely client-side: JavaScript's Date object reads the system clock, converts to UTC, and applies each zone's offset (including current DST status) using the IANA tz database that ships with the browser. The setInterval timer fires once per second to refresh the display; under the hood, browsers schedule timers with millisecond precision but throttle inactive tabs to once per second or slower to conserve battery. The visible second-hand drift relative to the system clock is typically under 100 ms.

A worked example: at 14:30:00 UTC on November 5 (after US DST ends), the display might show New York 09:30 (EST UTC−5), London 14:30 (GMT UTC+0, observing DST end the previous Sunday), Tokyo 23:30 (JST UTC+9, no DST), Sydney 01:30 next day (AEDT UTC+11, observing southern-hemisphere DST). The hour-and-minute offsets are stable across all dates in the same DST regime; the date label shifts when local time crosses midnight. Sydney being in the next calendar day from UTC at this moment is the typical 'tomorrow already started' scenario for east-of-prime-meridian zones.

Limitations are about clock accuracy and timer precision. The display reflects the user's system clock, not an authoritative time source. If the OS clock is wrong (drifted, manually misset, NTP sync failed), every displayed zone is wrong by the same amount. macOS and Windows typically sync via NTP every few hours; long-uptime laptops can drift by seconds or minutes. For applications requiring authoritative time, query an NTP server (time.google.com, time.cloudflare.com) or a public time-API; for casual display, the system clock is usually accurate to within a few seconds.

DST transitions are handled correctly through the IANA tz database lookup. On the day of transition, the affected zone's clock jumps forward (spring) or back (fall) at the exact local moment of transition; other zones are unaffected. Historical timestamps (entering a past datetime) use the rules in effect at that moment; the IANA database tracks rule changes so a 2005 timestamp converts using 2005's DST rules, not 2026's.

For cross-time-zone meeting planning, displaying the same UTC moment in multiple zones (rather than asking users to mentally convert) prevents the frequent off-by-an-hour errors that come from DST asymmetry between regions. The tool serves this directly: pick a moment, see it expressed simultaneously in any selected set of zones.