Working across time zones is one of the most error-prone parts of scheduling and communication. Here’s how to handle it without making mistakes.
How time zones work
The world is divided into 24 primary time zones, each roughly 15 degrees of longitude wide. UTC (Coordinated Universal Time) is the base reference. Other zones are expressed as offsets:
- UTC+0: London (GMT), Lisbon, Accra
- UTC-5: New York (EST), Toronto, Bogota
- UTC+1: Berlin, Paris, Lagos
- UTC+8: Beijing, Singapore, Perth
- UTC+9: Tokyo, Seoul
- UTC+5:30: India (IST) — yes, half-hour offsets exist
- UTC+5:45: Nepal — and quarter-hour offsets too
Daylight saving time (DST)
DST makes everything harder. When it’s in effect:
- US Eastern: UTC-5 → UTC-4 (EDT)
- US Pacific: UTC-8 → UTC-7 (PDT)
- Central Europe: UTC+1 → UTC+2 (CEST)
Key problems with DST:
- Not all countries observe it
- Start and end dates differ by region
- Some regions within the same country skip it (e.g., Arizona in the US)
- The southern hemisphere has opposite DST timing
Common conversion reference
| City | Standard | DST |
|---|---|---|
| London | UTC+0 | UTC+1 (BST) |
| New York | UTC-5 (EST) | UTC-4 (EDT) |
| Los Angeles | UTC-8 (PST) | UTC-7 (PDT) |
| Tokyo | UTC+9 | No DST |
| Sydney | UTC+10 (AEST) | UTC+11 (AEDT) |
| Dubai | UTC+4 | No DST |
| New Delhi | UTC+5:30 | No DST |
Scheduling across time zones
The overlap problem
A team in New York (EST), London (GMT), and Tokyo (JST) has very limited overlap:
NY London Tokyo
08:00 08:00 13:00 22:00
09:00 09:00 14:00 23:00
10:00 10:00 15:00 00:00 (next day)The only reasonable meeting time is 8-10 AM Eastern, which is already evening in Tokyo.
Best practices
- Always specify the timezone — “Meeting at 3 PM” is useless without a timezone. Say “3 PM EST” or better, “3 PM UTC-5”
- Use UTC for systems — Store and transmit times in UTC, convert to local only for display
- Use ISO 8601 —
2024-03-09T15:00:00-05:00is unambiguous - Account for DST transitions — A recurring 9 AM meeting might shift by an hour when DST changes
Time zone abbreviations are ambiguous
- CST = Central Standard Time (UTC-6) OR China Standard Time (UTC+8)
- IST = Indian Standard Time (UTC+5:30) OR Irish Standard Time (UTC+1) OR Israel Standard Time (UTC+2)
- EST = Eastern Standard Time (UTC-5) — but is it US or Australia?
Use UTC offsets or full timezone names (like America/New_York) to avoid confusion.
The International Date Line
Crossing from UTC+12 to UTC-12 means jumping a full day. Tonga (UTC+13) and American Samoa (UTC-11) are only 100km apart but 24 hours different.
This creates fun facts:
- Samoa skipped December 30, 2011 entirely when they moved from UTC-11 to UTC+13
- Kiribati has UTC+14, the furthest ahead — they’re the first to see each new day
Tools for time zone work
A timezone converter with live clocks helps you:
- See current time in multiple cities at a glance
- Convert a specific time between zones
- Plan meetings with visible overlap windows
No mental math needed — enter the time, pick the zones, and get instant results.