How to Use This Tool
Pick a date, a time and a starting city, then add the cities you care about. Every row is computed for that exact date, so daylight saving is already accounted for.
Why the date matters as much as the time
A timezone is not an offset. It is a set of rules for what the offset is on any given date. "New York" means UTC-5 in January and UTC-4 in July, and the switch happens on specific Sundays that themselves move. Convert 3 p.m. New York to London without knowing the date and you have a 50/50 chance of being an hour out.
This is why the tool asks for a date first. The same wall-clock time in the same city is a different UTC instant depending on which side of the daylight-saving boundary it falls.
Offsets are not all whole hours
The assumption that every zone is some number of hours from UTC is wrong, and confidently so:
- India is UTC+5:30 — the whole country, one half-hour offset.
- Nepal is UTC+5:45, fifteen minutes ahead of India, which is why a flight schedule between them looks like a typo.
- The Chatham Islands are UTC+12:45, and +13:45 under daylight saving.
- Lord Howe Island changes its clocks by thirty minutes, not an hour, when daylight saving starts.
All four were checked against the browser's own timezone data before being written here. A fixed-offset table cannot represent any of them.
How this stays correct when the rules change
Countries change their timezone rules more often than you would think — a government moves a daylight-saving date, or abolishes it, sometimes with weeks of notice. This tool does not carry its own table. It asks the browser, and the browser ships the IANA timezone database that is maintained centrally and updated with the operating system. So the accuracy of a future date depends on how current your device is, which is the honest limit and worth knowing: a date years out assumes today's rules still hold.
A practical habit for scheduling
When you send an invite across zones, name the zone explicitly — "3 p.m. New York time" — rather than just "3 p.m.". The words remove the ambiguity that a bare number leaves, and they survive being forwarded to someone whose calendar is set to somewhere else. For anything recurring, UTC is the anchor that never drifts.
