Time zone calculator
One time, every city that matters. Drag the needle across the board and watch who is awake. Nothing leaves the page. Your browser already knows every time zone.
Add a city or two and the board lights up where everyone is at their desk.
The board is empty. Add a city below.
1 ZONE OVERLAP 8 H UTC-7
How to read the board
Every row is one zone, drawn across the same day: the day in the zone you start from, midnight to midnight. A lit cell is an hour between 9 in the morning and 5 in the afternoon in that zone's own local time. The dark cells are its night. Where a row's day flips over, a heavy line marks its midnight, and the readout on the right says +1 or -1 so nobody books a call on the wrong date.
The column that stays lit in every row is the hour that works for everyone. It gets a frame, and the line under the board counts it. Drag the needle into it, press Copy as text, and paste the result into the invite.
What you can type
A city (Tokyo, Lisbon, Denver), a zone name (Europe/Berlin), or the abbreviations people actually use: PT, MT, CT, ET, GMT, BST, CET, EET, IST, GST, SGT, HKT, JST, KST, AEST, NZST, and about fifty more. Abbreviations are ambiguous in real life, so IST means India here and CST means Chicago; type the city if you mean a different one.
Daylight saving and offsets
Each row shows the zone's UTC offset at that exact moment, the local abbreviation, and DST when daylight saving is in effect. Pick a date in March or October and watch a row shift by an hour while its neighbour stays put. That is the whole reason this page exists.
What stays here
Everything on this page is plain JavaScript in your browser. There is no server doing the conversion, so there is nothing to send it to. The zones you add are kept in this browser's storage and nowhere else; a shared link carries the time and the zone names in its address and nothing more. The only thing this page records is that a button was pressed, and how many zones were on the board.