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Calculate the time difference between any two UTC offset time zones worldwide. Find what time it is in another city, plan international meetings, and account for daylight saving time.
24 primary time zones each represent one hour of Earth rotation (15° longitude). Political boundaries make zones irregular. About 40% of countries do not observe DST — including China, Japan, India, Russia, and most of Africa and Middle East.
| Cities | Difference | Example |
|---|---|---|
| New York↔London | 5 hrs | 9am NY = 2pm London |
| London↔Dubai | 3 hrs | 9am London = 12pm Dubai |
| London↔Singapore | 8 hrs | 9am London = 5pm Singapore |
| NY↔Tokyo | 13-14 hrs | 9am NY = 10pm Tokyo |
| Sydney↔London | 10-11 hrs | 9am London = 7pm Sydney |
Daylight Saving Time: USA springs forward 2nd Sunday March, falls back 1st Sunday November. EU: last Sunday March to last Sunday October. UTC (Coordinated Universal Time) is the international standard maintained by atomic clocks. Practically identical to GMT (within 0.9 seconds).
Earth is not a perfect sphere but an oblate spheroid — slightly flattened at the poles with an equatorial radius of 6,378,137 m and polar radius of 6,356,752 m. The difference (about 21 km) is small enough that treating Earth as a sphere introduces only about 0.3% error for most geographic calculations. The Haversine formula uses a mean radius of 6,371 km. For precision surveying requiring millimeter accuracy, the Vincenty formula using the full ellipsoid model is required.
While Earth's 24-hour day suggests 24 simple time zones each 15 degrees wide, political and geographic factors create significant complexity. The world actually has 38 distinct time zone offsets — some countries use 30-minute or even 45-minute offsets from UTC. China spans 5 geographic time zones but uses a single national time (UTC+8). Russia spans 11 time zones. Indiana (USA) historically had different rules in different counties. Daylight saving time transitions create two periods per year when the effective offset between any two locations may temporarily change.