A meeting at 09:00 in New York — what time is that in London, and how long is the meeting? Those are two different time problems, and two different calculators solve them. The time zone calculator converts one instant into another zone’s local clock and calendar. The time duration calculator measures the gap between two clock readings. Converting a time tells you when something is; measuring a duration tells you how long something lasts.
The genuine choice is which question you are answering. For handoffs, flights, webinars, and deadlines you need the instant — the zone converter. For run sheets, work logs, billing, and workouts you need the interval — the duration calculator. A full plan often needs both: convert the endpoints into one context, then measure the span between them.
What each calculator does
The time zone calculator takes a source IANA zone, a local date, a 24-hour time (00:00–23:59), and a target IANA zone. It maps the source entry to one instant, then formats that same instant in the target zone, reporting the converted time, converted date, offset difference, and daylight-saving status. The offset difference is computed from the two zones’ offsets at that instant: (target offset − source offset), rounded to the nearest minute, with the sign retained. In its checked example, 09:00 America/New_York on 2026-06-25 becomes 14:00 Europe/London, a +5:00 difference, because New York is UTC−4 and London is UTC+1 that day. The converted date can roll across the international date line.
The time duration calculator converts every clock time to seconds after midnight, then supports three modes. Difference mode subtracts start from end and adds one 24-hour day (86,400 seconds) if the end is earlier, so 13:30:00 to 17:45:00 yields 15,300 seconds, or 4 hours 15 minutes, with 4.25 total hours and 255 total minutes. Add and subtract modes combine an interval in days, hours, minutes, and seconds, then wrap the result modulo 86,400 into a time of day. The page is clock-only: no time zones, no dates, no daylight-saving days.
Side-by-side comparison
| Time zone calculator | Time duration calculator | |
|---|---|---|
| Question answered | What is this instant in another zone? | How long is this interval? |
| Primary inputs | Source zone, date, time; target zone | Two clock times, or a base time plus interval |
| Core method | Format one instant under each zone’s IANA rules | Seconds-after-midnight arithmetic; ±86,400 adjustment |
| Handles DST? | Yes — per-date offsets and daylight-saving labels | No — clock-only, no transition modeling |
| Handles dates? | Yes — target date can roll across the date line | No — difference mode adds one day at most |
| Typical outputs | Converted time and date, offset difference | Days, hours, minutes, seconds; unit totals |
When to use which
Use the time zone calculator whenever the position of a moment matters: sending a meeting time, catching a flight, coordinating a release, or checking a deadline in another jurisdiction. Write the result with the date and both zone names — “09:00 America/New_York / 14:00 Europe/London on 2026-06-25” — and remember the converted date is part of the answer.
Use the time duration calculator whenever the length of an interval matters: a work log, a study block, an audio edit, a workout, or a support window. It is also the right tool after a conversion, once both endpoints live in the same zone. Be careful with payroll: the duration page has no rounding policy and does not decide whether a break is paid, and wall-clock subtraction is wrong on daylight-saving transition days.
Where to start
- Time zone calculator — convert an instant between IANA zones with date and DST context.
- Time duration calculator — measure intervals between clock times, or add and subtract time.
Informational note: This page is an educational comparison, not timekeeping, payroll, legal, or financial guidance. Time-zone conversions depend on IANA rules that change over time, and clock-only durations ignore zones, dates, and daylight-saving days. Verify jurisdictional deadlines and payroll rules against authoritative sources.