Why timezone math goes wrong
Most scheduling mistakes come from three traps:
- Daylight saving time shifts on different dates. The US moves its clocks about two weeks before Europe in spring. For those two weeks, the usual "9am New York = 2pm London" rule is off by an hour — and half your standing meetings quietly break.
- Not every offset is a whole hour. India is UTC+5:30, Nepal is UTC+5:45, and parts of Australia use half-hour offsets. Rounding to the nearest hour means being 30 minutes late.
- Countries change the rules. Governments regularly move, abolish, or introduce DST. Any offset you memorized last year may be wrong this year.
The fix is to never compute offsets in your head. Always read the current local time for each city from a tool that tracks the IANA timezone database — that's what a dedicated timezone app (or even a quick web search) gives you that mental arithmetic doesn't.
Step 1: Find the working-hours overlap
Instead of asking "what time is it there?", ask "when are we both at work?". For each participant, block out their local 9:00–17:00, then look for the window where those blocks intersect:
| City | Their 9–17 | In your time (London) |
|---|---|---|
| San Francisco | 9:00–17:00 PT | 17:00–1:00 |
| London | 9:00–17:00 | 9:00–17:00 |
| Singapore | 9:00–17:00 SGT | 1:00–9:00 |
In this example there is no shared working-hours window at all — which is itself the answer: someone has to flex. London–San Francisco overlaps 17:00–18:00 UK time; London–Singapore overlaps 9:00–10:00 UK time. A three-region call has to land in someone's early morning or evening.
Step 2: Be deliberately unfair — in rotation
When there's no perfect slot, don't let the same region eat the 7am call every week. The teams that make global meetings sustainable rotate the pain: this month the early slot hits Asia, next month the US, then Europe. Put the rotation in the calendar invite itself so nobody has to renegotiate it.
Step 3: Schedule in the other person's local time
When proposing times, write them in the recipient's local time ("Would 9:30 your time on Thursday work?"). It removes ambiguity, shows respect, and eliminates the classic "wait, whose 3pm?" thread. Calendar tools convert automatically once the event exists — the risky moment is always the plain-text proposal before that.
Step 4: Protect the quarter hours
Half-hour timezone offsets mean "on the hour" for you can be "on the half hour" for Mumbai. If you work with South Asia regularly, plan in 15-minute increments rather than hours — a slider with 15-minute steps (like the one in GlobalTimeZones) makes the :30 and :45 offsets visible instead of surprising.
Quick checklist before you hit send
- Did you check today's offset, not the one you remember? (DST!)
- Is the time inside — or at least adjacent to — everyone's working hours?
- If someone gets a bad slot, is it their turn?
- Did you state the time in each recipient's local time?
- Is there an async alternative (recorded update, doc review) for people at 3am?
Tools that help
You don't need heavyweight software. Three good options:
- A menu bar world clock — always-visible local times for your key cities, with working-hours indicators. See our guide to setting up a world clock on your Mac.
- Your calendar's secondary timezone — both Google Calendar and macOS Calendar can show a second timezone column in week view.
- Scheduling links — for external meetings, tools like Calendly render your availability in the invitee's local time automatically.