How to work out a time difference
Working out the time in another city is a matter of comparing two offsets from the same reference clock. Once you know how, you can do it in your head, and you know where the traps are.
Start from UTC
Coordinated Universal Time (UTC) is the world's reference clock. Every time zone is described as an offset from it: Seoul is UTC+9, Paris is UTC+1 in winter, Los Angeles is UTC−8 in winter. The offset tells you how many hours to add to UTC to get the local time.
Subtract the offsets
The difference between two cities is simply the difference between their offsets. Seoul (UTC+9) minus Paris (UTC+1) is eight hours, so Seoul is eight hours ahead: when it is 18:00 in Seoul, it is 10:00 in Paris. Seoul minus Los Angeles (UTC−8) is 9 − (−8) = 17 hours, so 18:00 in Seoul is 01:00 the same day in Los Angeles.
Watch the date
When the difference pushes the time past midnight, the date changes too. A call at 09:00 on Monday in Sydney is still Sunday afternoon in Los Angeles. Near the International Date Line the effect is extreme: Samoa (UTC+13) and American Samoa (UTC−11) are only a short flight apart but on different calendar days, 24 hours apart.
Not every zone is a whole hour
Some countries chose offsets that are not whole hours. India uses UTC+5:30, Iran UTC+3:30, Nepal UTC+5:45, central Australia UTC+9:30 and the Chatham Islands of New Zealand UTC+12:45. The method is the same; just keep the minutes: Kathmandu (UTC+5:45) is 3 hours 15 minutes behind Tokyo (UTC+9).
Daylight saving time changes the offset
An offset is only valid for a period of the year. In summer Paris moves to UTC+2 and Los Angeles to UTC−7, but Seoul stays at UTC+9, so the Seoul–Paris difference shrinks to seven hours. Always use the offsets for the date in question, not today's. Our daylight saving guide explains when clocks change.
A quick checklist
- Find each city's offset for the date you care about.
- Subtract: positive means the first city is ahead.
- Add the difference to the known time and check whether the date rolls over.
- Watch for half-hour and 45-minute zones.
Or skip the arithmetic: every city page on the world clock lists its difference from major cities, already adjusted for daylight saving time.
More guides: Daylight saving time, explained · Scheduling meetings across time zones · Reading the day and night map