Every conversion is just the difference in UTC offsets
Every zone is defined by how far it is ahead of or behind UTC. To move a time from one zone to another, subtract the source offset and add the target offset. A location further east has the larger offset and the later clock. The only thing that changes an offset during the year is daylight saving.
UTC offsets for major cities
| City | Zone | Standard | Daylight |
|---|---|---|---|
| Los Angeles | PT | UTC−8 | UTC−7 |
| New York / Toronto | ET | UTC−5 | UTC−4 |
| London | GMT / BST | UTC+0 | UTC+1 |
| Paris / Berlin | CET | UTC+1 | UTC+2 |
| Mumbai (India) | IST | UTC+5:30 | — |
| Kathmandu (Nepal) | NST | UTC+5:45 | — |
| Tokyo | JST | UTC+9 | — |
| Adelaide (S. Australia) | ACST | UTC+9:30 | UTC+10:30 |
| Sydney | AET | UTC+10 | UTC+11 |
Zones like India, Nepal and Japan stay on one offset all year. The world runs from UTC−12 to UTC+14, so the widest gap between two clocks is 26 hours.
The two things that catch people out
- Daylight saving shifts the offset. When a zone is on summer time its clock jumps an hour, so New York moves from UTC-5 to UTC-4. Use the offset that applies on the actual date, not the standard one.
- Hemispheres and start dates don't line up. The US, Europe and Australia switch on different weekends, and the southern hemisphere runs summer time during the northern winter. For a week or two each spring and autumn the usual gap between two cities is off by an hour.
- Some countries dropped daylight saving. Most of Mexico, Brazil, Russia and Turkey now hold a fixed offset year-round, so their gap to Europe or the US moves twice a year even though their own clock never changes.
Common questions
How do I convert a time between two zones?
Take the difference of their UTC offsets and add it to the source time. If a city is at UTC-5 and you want UTC+1, the target is 6 hours ahead (1 minus -5). Going east adds hours, going west subtracts them.
Is UTC the same as GMT?
For everyday purposes, yes. Both sit on the Greenwich meridian at zero offset. UTC is the modern atomic-clock standard used by computers and aviation; GMT is the older civil term. The clock reading is the same.
Why do some countries use a 30 or 45 minute offset?
A handful of places sit between whole-hour meridians and chose a fractional offset to match local solar noon more closely. India is UTC+5:30, Nepal UTC+5:45, and parts of Australia run UTC+9:30.
