Time Zone Glossary

The global atomic time standard — the baseline all other timezones are offset from. Never changes for Daylight Saving Time.

The UK's standard timezone in winter, based on solar time at the Greenwich meridian. Replaced by UTC as the international standard in 1972.

UTC and GMT show the same clock time right now, but UTC is an atomic standard while GMT is a timezone. In summer, the UK switches to BST (UTC+1), not GMT.

The practice of moving clocks forward in spring and back in autumn to shift daylight to evening hours. Not observed by India, China, Japan, UAE, and most of Africa.

A region that observes a uniform standard time. There are 38 distinct UTC offsets in use worldwide — more than the theoretical 24 due to half-hour and quarter-hour zones.

The number of seconds elapsed since January 1, 1970 UTC. The universal way computers store timestamps internally — the same in every timezone.

The international standard for writing dates and times as strings: YYYY-MM-DDTHH:MM:SSZ. Eliminates ambiguity between date formats like 04/06 (April 6 or June 4?).

The public dataset almost every computer and phone uses to convert time zones correctly, using names like America/New_York instead of ambiguous abbreviations.

A one-second correction occasionally added to UTC to keep it aligned with the Earth's rotation. Unix time ignores it completely.

CST alone could mean US Central time, China, or Cuba. IST could mean India, Israel, or Ireland. Here's why, and the actual fix.

Solar time follows the sun's actual position. Civil (clock) time follows your time zone's standardized offset — and the two can be an hour or more apart.

Zulu time is UTC by another name, used in aviation, the military, and maritime navigation. "1430Z" means 2:30 PM UTC, everywhere.