Time Zone Glossary
Plain-English definitions for UTC, GMT, DST, Unix time, ISO 8601, and other terms that come up constantly in time zone work. No jargon — just clear explanations.
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?).
Looking to convert times rather than look up definitions? Time Zone Converter · World Clock · Blog