How the conversion works
Enter a time as it would appear on a clock in city A, and the calculator finds the same absolute instant in city B. It first converts your wall-clock time to a single point in time (a Unix timestamp), then renders that instant on the clock of city B using the IANA time zone database built into your browser. The difference is the UTC offset of B minus the UTC offset of A at that exact instant.
Worked example
City A: New York, 14:00 on Oct 10. City B: Tokyo.
- 14:00 in New York (EDT, UTC-4) is 18:00 UTC.
- 18:00 UTC is 03:00 the next day in Tokyo (JST, UTC+9).
- Difference: +9 - (-4) = +13 h. Tokyo is ahead of New York.
Try Kolkata for a half-hour case: 14:00 in New York is 23:30 the same day in Kolkata (IST, UTC+5:30), a difference of +9 h:30.
Daylight saving time is handled automatically
Offsets are measured at the instant you picked, not from a fixed table, so the tool follows daylight saving changes. New York is UTC-5 in January and UTC-4 in July, and the calculator knows which one applies to your date. Cities without daylight saving, like Tokyo or Singapore, stay on the same offset all year.
Why the difference can change over the year
Two cities can drift apart and back together as their daylight saving schedules change. The United States and Europe switch on different weekends, so for a few weeks each spring and autumn the difference between New York and London is 4 hours instead of the usual 5. Always convert for the actual date of your call or flight, not from a number you remembered.