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. Each term has its own page; this one shows how they relate and which one to open first.
One Moment, Six Ways of Writing It
Most of the terms below are different ways of naming or writing the same instant. Take 2:30 PM UTC on June 11, 2026:
| Written as | What it tells you | Read more |
|---|---|---|
| 14:30 UTC | The time on the global standard, with no daylight saving | UTC |
| 1430Z | The same, in the aviation and military style | Zulu time |
| 2026-06-11T14:30:00Z | The same, as a machine-readable string | ISO 8601 |
| 1781188200 | The same, as seconds since January 1, 1970 | Unix time |
| 10:30 AM EDT | New York in summer, under a daylight saving label | DST |
| 8:00 PM IST | India, where the abbreviation could also mean Israel or Ireland | Abbreviation ambiguity |
Where to Start
- Time standards: start with UTC, then UTC vs GMT. The leap second page explains why UTC is not a perfectly smooth count.
- Zones and clock changes: Time zone and DST first. The solar vs civil time page covers why zone borders look the way they do.
- Writing and storing times: ISO 8601 for strings, Unix time for numbers, and the IANA database for zone names in software.
The global time standard, computed from atomic clocks. The baseline all other timezones are offset from, and it never changes for Daylight Saving Time.
The UK's standard time in winter, defined as mean solar time at Greenwich and equal to UTC+0. UTC replaced it as the international civil time standard in 1972.
UTC and GMT show the same clock time, but UTC is computed from atomic clocks while GMT is solar time at Greenwich. 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. Nearly 40 distinct UTC offsets are in use worldwide, more than the theoretical 24 because of half-hour and 45-minute 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, and leap seconds are being phased out.
CST alone could mean US Central time, China, Cuba, or central Australia. 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 weather forecasting. "1430Z" means 2:30 PM UTC, everywhere.
Looking to convert times rather than look up definitions? Time Zone Converter · World Clock · Blog