Time zones allow the world to coordinate activities across geographic regions. Understanding how to convert between time zones is essential for international business, travel, and communication.
How Time Zones Work
The world is divided into 24 main time zones, each approximately 15° of longitude wide (360° ÷ 24 hours). All zones are expressed as offsets from UTC (Coordinated Universal Time):
Local time = UTC + offset
Example: If UTC is 14:00:
- New York (UTC−5): 09:00
- London (UTC+0): 14:00
- Dubai (UTC+4): 18:00
- Tokyo (UTC+9): 23:00
Common Time Zone Offsets
| City | Zone | UTC offset | DST |
|---|---|---|---|
| London | GMT/BST | 0 / +1 | Yes |
| Paris, Berlin | CET/CEST | +1 / +2 | Yes |
| Dubai | GST | +4 | No |
| Mumbai | IST | +5:30 | No |
| Beijing, Singapore | CST/SGT | +8 | No |
| Tokyo | JST | +9 | No |
| Sydney | AEST/AEDT | +10 / +11 | Yes |
| New York | EST/EDT | −5 / −4 | Yes |
| Chicago | CST/CDT | −6 / −5 | Yes |
| Los Angeles | PST/PDT | −8 / −7 | Yes |
Converting Between Time Zones
Formula:
Target time = Source time + (Target offset − Source offset)
Example: It is 10:00 AM in London (UTC+0). What time is it in Tokyo (UTC+9)?
Tokyo time = 10:00 + (9 − 0) = 10:00 + 9:00 = 19:00
Crossing midnight: If the result exceeds 24:00, subtract 24 and add one day. If it goes negative, add 24 and subtract one day.
Daylight Saving Time (DST)
Many countries shift clocks forward by 1 hour in summer:
UK:
- DST starts: last Sunday in March (clocks forward 1 hour)
- DST ends: last Sunday in October (clocks back 1 hour)
- During summer: UK = UTC+1 (BST)
US:
- DST starts: second Sunday in March
- DST ends: first Sunday in November
Countries without DST: Japan, China, India, UAE, most of Africa.
The International Date Line
The 180° meridian (roughly in the Pacific Ocean) is the International Date Line. Crossing it westward: add one day. Eastward: subtract one day.
A flight from Los Angeles (Tuesday 11:00 PM) to Tokyo (13 hours):
Departure: Tuesday 23:00 (UTC−7) = Wednesday 06:00 UTC
Flight time: 13 hours
Arrival UTC: Wednesday 19:00 UTC
Tokyo local: Wednesday 19:00 + 9 = Thursday 04:00
Tips for International Scheduling
- Always specify the time zone when scheduling (e.g. "14:00 UTC" or "2 PM EST")
- Use tools like World Time Buddy or Google Calendar for multi-zone scheduling
- India (UTC+5:30) and Nepal (UTC+5:45) use half-hour offsets — easy to miss