Time Zone Difference Calculator
Calculate the time difference between two cities.
Disclaimer: The results provided by this tool are estimates for informational purposes only. Actual values may vary. Please verify important calculations independently.
How the hour gap is computed
The time zone difference between two locations is their UTC offsets subtracted from each other. Tokyo sits at UTC+9, London at UTC+0 during winter, so Tokyo is 9 hours ahead. When London switches to British Summer Time at UTC+1, the same gap narrows to 8 hours. The tool computes each zone's offset for the specific date you are asking about, including any daylight saving adjustment in effect that day, then returns the signed difference. Because the calculation flows through UTC rather than comparing wall-clock readings, it works the same way whether the two zones are both ahead of UTC, both behind, or straddling it.
Fractional offsets are handled at the minute level. India at UTC+5:30, Iran at UTC+3:30, and Nepal at UTC+5:45 all produce differences that are not whole numbers of hours. The difference between Nepal and India, for instance, is exactly 15 minutes — a surprisingly common source of calendar errors for teams working across South Asia. The time zone difference tool expresses these fractions clearly rather than rounding to the nearest hour, so when a meeting invite says 10:00 AM India time, you can see that it is 10:15 AM in Kathmandu rather than discovering the mismatch when someone joins late.
When the difference changes across the year
Any pair of zones where one or both observe daylight saving will show a difference that varies by season. New York and London are 5 hours apart most of the year, but briefly become 4 hours apart in the two weeks after the US springs forward and before European summer time begins, and again in the fall when the US falls back before Europe does. Sydney and London can be anywhere from 9 to 11 hours apart depending on which hemisphere is currently on summer time. If you schedule a recurring meeting across a DST boundary, expect the local time on one side to shift by an hour twice a year even though the other side never moves.
A time zone difference is also not the whole story for scheduling. A 9-hour gap between Tokyo and London means there is almost no overlap between standard business hours on both sides; the only realistic live meeting windows are early morning in Tokyo with late evening in London, or vice versa. For teams working across large gaps, asynchronous handoffs often beat synchronous meetings. And remember that date changes come into play: when it is 10 AM Monday in Tokyo, it is already 1 AM Monday in London but 8 PM Sunday in New York. The tool shows the hour difference cleanly, but the calendar date is often the first thing people miss.