World Clock — Multi-Timezone With Live Day/Night Terminator Map

A Brief History of World Time

Before 1884 every city kept its own local solar time — Paris was 9 min 21 sec ahead of London, Boston 11 min ahead of New York. The railway boom of the 1840s made this untenable: a Boston-bound train could not be dispatched safely if every station along the route reset its clock to local noon. Britain adopted Greenwich Mean Time (GMT) for railway schedules in 1847; the US followed with five "railroad zones" on November 18, 1883 (the "day of two noons").

The 1884 International Meridian Conference in Washington DC, attended by 25 nations, ratified Greenwich as the prime meridian and split the world into 24 one-hour zones. Sandford Fleming, the Canadian railway engineer who proposed the scheme in 1879, is the father of standard time. France held out until 1911, insisting Paris was the prime meridian; the agreement to switch was nicknamed the "polite British surrender of meridional pride".

In 1972 UTC formally replaced GMT as the world's primary time standard, defined by the BIPM (Bureau International des Poids et Mesures) using a weighted average of about 400 atomic clocks across 70 timing laboratories. The International Atomic Time scale (TAI) drifts about 1 second per 100 million years; UTC is TAI adjusted by leap seconds (37 leaps inserted between 1972 and 2017) to stay within 0.9 seconds of solar time. Leap seconds will be discontinued after 2035 per a 2022 CGPM resolution.

The terminator — the line separating day from night — is a great circle perpendicular to the Sun-Earth axis. On a flat equirectangular projection it draws as a sinusoid because of the Mercator distortion. The 23.45 deg axial tilt of Earth means the subsolar point shifts seasonally: near the summer solstice (Jun 21), the sun is overhead at 23.45 deg N, leaving the entire Antarctic in continuous polar night. Today (May 27) the declination is about +21.5 deg N — Northern Hemisphere late spring.

IANA Time Zone Database (also called tzdata or zoneinfo) is the authoritative source for civil time zones worldwide. Maintained by Paul Eggert at UCLA since 2011 (originally created by Arthur David Olson at NIH in 1986), it tracks every DST transition, political renaming, and historical offset for every region back to 1970. The 2026a release (April 2026) included an update for the Bishkek and Ulan-Bator timezones.

Modern aircraft and ship navigation systems use UTC exclusively. Aviation pilots refer to UTC as "Zulu time" (Z) from the NATO phonetic alphabet — "0900Z" is 09:00 UTC. ICAO mandates Zulu time on all flight plans and ATC clearances. The world clock you see here is a 21st-century version of the navigator's chronometer — the same problem (knowing what time it is everywhere), solved with JavaScript instead of brass and pivot jewels.

For converting a single specific time between two zones see Time Zone Converter; for a single-zone live display use the Current Time tool.