27.7°N 85.3°EKathmandu, Nepal
Earth turns 15 degrees an hour
Earth turns once on its axis in about 24 hours, so 360 degrees divided by 24 gives 15 degrees of longitude per hour, or 4 minutes per degree. When it is solar noon in one place, it is solar noon one hour later 15 degrees to the west. This is the logic behind standard time zones, each about 15 degrees wide in theory.
From sun time to standard time
Towns once kept their own local time from the sun, so neighboring cities could differ by several minutes. That worked until railroads needed timetables. Railroads in the United States and Canada adopted standard time zones in 1883, and the 1884 meridian conference set Greenwich as the zero line for longitude. Countries adopted zones gradually after that.
UTC and offsets
Coordinated Universal Time (UTC) is the reference. Every zone is written as an offset from it, such as UTC-5 or UTC+9. Offsets run from UTC-12 to UTC+14, a range wider than 24 hours because some Pacific island nations moved to the far side of the date line.
| Place | Standard offset |
|---|---|
| Los Angeles | UTC-8 |
| New York | UTC-5 |
| London | UTC+0 |
| India | UTC+5:30 |
| Nepal | UTC+5:45 |
| China | UTC+8 |
| Tokyo | UTC+9 |
| Sydney | UTC+10 |
Half-hour zones and political lines
Zones follow national borders more closely than meridians. India uses a single zone at UTC+5:30, Newfoundland in Canada uses UTC-3:30, central Australia uses UTC+9:30 and Nepal uses UTC+5:45. China spans roughly 60 degrees of longitude, about four zones' width in theory, but uses one time, UTC+8. In its far west, the sun can rise very late by the clock.
The International Date Line
Go around the world and you add or lose a full day, so the calendar needs a line where the date changes. The International Date Line runs roughly along 180° longitude through the Pacific, with zigzags so that island groups share a date. Crossing it westward, you skip ahead a day. Crossing eastward, you repeat one.
Daylight saving time
Many countries move their clocks forward an hour in summer, but many others never do, and the start and end dates differ between countries and between hemispheres. The rules also change from time to time by law. Before you schedule a call or a flight connection, check the current rules for both places on the exact dates.
How to convert between time zones
- Write down each place's current offset from UTC, including daylight saving if it applies on that date.
- Convert the known time to UTC by subtracting its offset. 9:00 in New York at UTC-5 is 14:00 UTC.
- Add the other place's offset. Tokyo at UTC+9: 14:00 + 9 hours = 23:00 the same day.
- Check whether the date changed. If the result goes past midnight, it is the next day there.
When you share a meeting time across zones, quote it in UTC or with the offset, such as 14:00 UTC. Names like "Eastern" or "CST" can mean different things in different countries.
Time zone FAQ
How many time zones are there?
There are 24 one-hour zones in theory, but more than that in use once you count half-hour and 45-minute offsets and zones beyond UTC+12. The exact count depends on how you count them and changes as countries adjust their clocks.
Is UTC the same as GMT?
For everyday purposes they show the same time. GMT is also the name of the UK's winter time zone, while UTC is the scientific reference standard, kept by atomic clocks.
Why does the date line zigzag?
So that countries made up of scattered islands, and neighbors that trade closely, stay on the same calendar day.