An overseas meeting at "9 AM their time", a video call with family across the world, an international flight — get the time difference wrong and you miss the meeting or the plane. Time zone math is easier than it looks. Here's how it works.
How Time Zones Work
The globe is divided into 24 time zones, measured from the prime meridian (0° longitude). Zones to the east are ahead (UTC+), to the west behind (UTC−). Each zone you cross = 1 hour of difference.
UTC (Coordinated Universal Time) is the modern international standard; GMT (Greenwich Mean Time) is essentially equivalent. Any place's time = UTC + its zone offset.
The Conversion Formula
Target time = local time + (target zone offset − local zone offset)
Example: New York is UTC−5 and London is UTC+0, an 8-hour difference. When it's 3 PM in London, it's 7 AM in New York. Manual math gets confusing fast — just pick two cities in the time zone converter and get the matching time instantly.
The Daylight Saving (DST) Trap
Many regions (US, Europe, and others) observe daylight saving time, shifting clocks each year — so the difference between two cities changes across the year. The US-UK gap, for example, is 5 hours in winter and 4 hours in summer. Always confirm whether the other side is on DST.
Reference: Common City Offsets (UTC)
| City | Zone | vs London |
|---|---|---|
| London | UTC+0 | 0 |
| New York | UTC−5 | −5h |
| Los Angeles | UTC−8 | −8h |
| Tokyo | UTC+9 | +9h |
| Sydney | UTC+10 | +10h |
| Beijing | UTC+8 | +8h |
Four Tips for Cross-Zone Coordination
- Find the overlap: for transatlantic calls, aim for early-morning/late-evening windows both sides can handle
- State both times: when inviting, list everyone's local time so nobody miscalculates
- Add multiple clocks: put the key cities on your phone's clock widget
- Build in buffer: give cross-zone deadlines extra lead time
Beating Jet Lag
After a long flight, switch to local time immediately — get daylight, avoid daytime naps, and you'll typically adapt in 2-3 days. Before you travel, plan your landing schedule with the time zone converter so you ease in rather than crash.
For business or travel, run the time zone converter first — and stop getting your cross-time-zone plans wrong.