How to use it
- Click anywhere on the map for the zone at that point, its offset and the time there.
- Read the table of the time now in big cities. Click a row to fly to it.
- Read the colours: red at UTC−12, green at UTC, red again at UTC+12.
5 practical uses
- Find a meeting time that works across offices.
- Check the time before calling a friend or a customer abroad.
- See which zones a flight crosses.
- Learn where the half-hour and quarter-hour zones are.
- Check summer time in another country.
The maths behind it
The map is a raster: for each pixel the latitude is found from the inverse Mercator formula, φ = 2·atan(eʸ) − π/2, the time zone is looked up from a compressed quadtree of the IANA zone boundaries, and the colour is set by the zone's current offset from UTC on a hue circle. The offsets themselves come from the browser's time zone database.
About time zones
Solar time changes by four minutes for every degree of longitude. Railways in the 1880s replaced local solar time with hourly zones, in theory 15° wide, but countries draw their own: China keeps one zone across five nominal ones, India and Iran sit on the half hour, Nepal on the quarter, and Kiribati and Samoa chose to be first to the new day (UTC+14). Summer time shifts about seventy countries by an hour for part of the year, on dates that differ by country and hemisphere.
The zones are the IANA time zone database, evaluated in the browser with tz-lookup, on a grid fine enough for a world view; the edges are approximate at street scale.