Pick a date, a source IANA timezone, and a destination IANA timezone, and the calculator returns the signed offset in hours between the two zones on that exact day. The sign tells you the direction: a positive number means the destination is ahead of the source, negative means behind. Because the result is computed from the IANA tz database, daylight saving transitions, half-hour zones such as Asia/Kolkata, and 45-minute zones such as Asia/Kathmandu all resolve correctly. Use it to read a calendar invite, plan a deadline across continents, or verify whether the shift you remember from last month still applies today. Need a wall-clock conversion as well? Run the time through the timezone converter.
Common use cases
- Reading the offset for a recurring meeting. A standing call between New York and Tokyo runs every week, and you want to know which weeks the gap is 13 hours and which are 14. Pick a Monday from each month and the calculator returns the offset on that specific date, so you can spot the DST flip in March or November before it bites the calendar invite.
- Setting a deadline that lands the same day everywhere. A submission portal closes at 17:00 in London. Convert that to your local zone by reading the offset, then schedule a reminder ahead of it. Pair the offset with the time duration calculator when the deadline is given as "24 hours from now" rather than a fixed local time.
- Comparing two zones that both observe DST. New York and London both shift to summer time, but on different dates: New York moves on the second Sunday in March, London on the last Sunday. For three weeks each spring, the gap is 4 hours instead of 5. Pick any date in that window and the calculator confirms it.
- Verifying a half-hour or quarter-hour offset. India is UTC+5:30, Nepal is UTC+5:45, Newfoundland is UTC-3:30, and parts of Western Australia run on UTC+8:45. The result rounds to the nearest fraction of an hour, so 5.5 or 5.75 is what you see, not a misleading whole number.
How it works
The calculator pins both zones to the same UTC instant on the chosen date (12:00 UTC), reads the offset of each zone from the IANA tz database, and returns the destination offset minus the source offset, expressed in hours. Picking 12:00 UTC keeps the result stable around the date's daylight-saving transitions, since both zones have already settled into their offset for that day by midday. The output is signed: positive when the destination is ahead, negative when it is behind.
- Pick the comparison date. Enter a calendar date as YYYY-MM-DD. The date matters because daylight saving and historical zone changes can shift the offset by an hour or more from one week to the next.
- Choose the source IANA zone. Select the zone you are starting from, such as America/New_York or Europe/Sarajevo. Use a named zone rather than a fixed offset so DST is read off the tz database for that day.
- Choose the destination IANA zone. Select where you want the comparison. Common picks are UTC, Europe/London, Asia/Tokyo, and Asia/Kolkata, but every zone in the IANA database is supported.
- Read the signed difference. The result shows the destination offset minus the source offset, expressed in hours and signed. A value of +13 means the destination is 13 hours ahead. A value of -5 means the destination is 5 hours behind.
Worked examples
New York to Tokyo on a US summer date
Compare America/New_York to Asia/Tokyo on 2026-04-27. New York is on EDT (UTC-4) and Tokyo is on JST (UTC+9) year-round.
Result: The calculator returns +13 hours.
During the US winter months, when New York is on EST (UTC-5), the same comparison returns +14 hours. The flip happens on the second Sunday of March and the first Sunday of November.
New York to London on the same summer date
Compare America/New_York to Europe/London on 2026-04-27. New York is on EDT (UTC-4) and London is on BST (UTC+1).
Result: The calculator returns +5 hours.
This is the steady gap for most of the year because both zones observe DST. The exception is the three-week window in spring when only New York has shifted, covered in the next example.
Cross-DST window between New York and London
Compare America/New_York to Europe/London on 2026-03-08. New York springs forward to EDT on this Sunday, but London stays on GMT until the last Sunday of March.
Result: The calculator returns +4 hours instead of the usual +5.
The gap snaps back to +5 on 2026-03-29 when London moves to BST. Plan trans-Atlantic meetings for this window using the local time you actually want, not a remembered offset.
UTC to Mumbai with a half-hour offset
Compare UTC to Asia/Kolkata on 2026-04-27. India runs on a fixed UTC+5:30 with no daylight saving.
Result: The calculator returns +5.5 hours.
Half-hour offsets are stored exactly in the IANA tz database, so the answer is the precise fractional value rather than a rounded whole number.
Honolulu to Tokyo across the date line
Compare Pacific/Honolulu to Asia/Tokyo on 2026-04-27. Honolulu is UTC-10 and Tokyo is UTC+9, both fixed year-round.
Result: The calculator returns +19 hours.
The pair never crosses a DST boundary, so this offset holds every day of the year. To translate a specific Honolulu wall-clock time, run it through the meeting planner for an overlap view.
Edge cases & gotchas
- Cross-DST transitions split the offset for weeks. When two zones both observe daylight saving but switch on different dates, the offset between them changes for the days in between. New York and London differ by 4 hours from the second Sunday of March to the last Sunday of March, and by 6 hours from the first Sunday of November to the last Sunday of October. Always enter the actual meeting date, not a typical date.
- Fractional offsets return decimal hours. Asia/Kolkata is UTC+5:30, Asia/Kathmandu is UTC+5:45, America/St_Johns runs on UTC-3:30, and parts of Western Australia observe UTC+8:45. The result rounds to the underlying fraction (5.5, 5.75, 8.75) rather than a misleading whole hour. Treat anything ending in .5 or .75 as a real offset, not a rounding artefact.
- Date-line crossings show as large offsets, not negative dates. Tokyo and Honolulu sit on opposite sides of the international date line, so the offset between them is +19 or -19 hours. The calculator never reports a negative date; it reports the time gap. To see which calendar day a specific clock time falls on, use the wall-clock converter.
- Same zone on different dates returns zero. Picking America/New_York for both source and destination returns 0 on every date, even on the spring-forward and fall-back days. The calculator compares offsets, and a zone has the same offset as itself by definition. Use this as a sanity check when wiring the form into a script.
Frequently asked questions about Time Difference Calculator
What does a positive number in the result mean?
A positive number means the destination zone is ahead of the source. +13 hours from America/New_York to Asia/Tokyo means Tokyo wall-clock time is 13 hours later than New York wall-clock time on that date.
Why does the offset change by date?
Daylight saving rules and historical zone changes move the offset. New York is UTC-5 in winter and UTC-4 in summer, so the gap to a non-DST zone like UTC differs by an hour depending on the date.
Why does the calculator pick noon UTC for the comparison?
Picking 12:00 UTC keeps the result stable around the day a zone shifts. Both zones have already settled into their offset for that calendar day by midday, so the math does not flicker as the transition crosses local midnight.
Does the tool handle fractional-hour offsets?
Yes. India returns 5.5, Nepal returns 5.75, Newfoundland returns -3.5 against UTC, and the tz database also encodes a 45-minute offset for the small Australian region of Eucla. The decimal is real and matches the local clock.
Why does the offset between New York and London change for three weeks in March?
New York moves to EDT on the second Sunday of March, but London stays on GMT until the last Sunday of March. During that window the gap is 4 hours instead of 5. The reverse happens in late October and early November when London shifts back to GMT before New York leaves EDT.
Can I compare two zones that both equal UTC?
Yes. The result is 0. Comparing UTC to UTC, or UTC to a fixed-offset zone of UTC+0 such as Atlantic/Reykjavik, returns 0 on every date.
What format is the answer in?
The answer is a signed number of hours, with one decimal place when the offset is fractional. +13, -5, +5.5, +5.75 are all possible results. The sign is always shown, including for positive values, so the direction is unambiguous.
Glossary
- IANA timezone
- A named region in the IANA tz database, such as America/New_York or Asia/Kolkata. The name encodes the full history of UTC offsets and daylight-saving rules for that region.
- UTC offset
- The signed difference between a local clock and Coordinated Universal Time, written as +HH:MM or -HH:MM. New York is UTC-5 in winter and UTC-4 in summer.
- Daylight saving time
- The seasonal one-hour shift of local clocks to extend evening daylight. Different countries shift on different dates, so the gap between two DST-observing zones can move for weeks at a time.
- Fractional offset
- A UTC offset that is not a whole number of hours. India runs on +5:30, Nepal on +5:45, Newfoundland on -3:30, and Eucla in Western Australia on +8:45.
- International date line
- The roughly meridional line in the Pacific where the calendar date changes. Two zones on opposite sides can be 19 to 26 hours apart in clock time even though they are physically close.