Quick reference

A time format that runs from 00:00 to 23:59, with no AM or PM. 14:00 means 2pm; 00:00 means midnight. Unambiguous across languages and cultures, making it the safest format for international communication.

A seasonal practice of moving clocks forward by one hour in spring and back in autumn, observed by many but not all countries. Because different countries change their clocks on different dates — and some never change them at all — a UTC offset that is correct today may be wrong next month.

Greenwich Mean Time — a time zone based at the Royal Observatory in Greenwich, London. GMT and UTC share the same value (UTC+0) and are often used interchangeably in everyday speech, but they are technically different: UTC is an international standard maintained by atomic clocks, while GMT is a geographic time zone. The UK observes GMT in winter and switches to BST (UTC+1) in summer.

A time zone identifier in the format Region/City (e.g. America/New_York, Europe/London). Maintained by the Internet Assigned Numbers Authority, this database encodes the complete history and future rules for every time zone in the world, including all daylight saving transitions. Every major operating system, calendar app, and programming language uses it under the hood.

The base UTC offset a region observes when daylight saving time is not in effect. For example, New York is UTC−5 in standard time (Eastern Standard Time) and UTC−4 during daylight saving (Eastern Daylight Time). Regions that do not observe daylight saving — such as India and most of Africa — are always on standard time.

A short code used to identify a time zone, such as EST, CST, or IST. Convenient but frequently ambiguous — CST can mean Central Standard Time (UTC−6) or China Standard Time (UTC+8), and IST can mean India, Ireland, or Israel. Prefer UTC offsets or IANA names when precision matters.

A number representing a moment in time as the count of seconds (or milliseconds) elapsed since 1 January 1970 00:00:00 UTC, known as the Unix epoch. Because it is always relative to UTC, a Unix timestamp has no time zone ambiguity. It is the most common format for storing moments in time in databases and APIs.

Coordinated Universal Time — the global time standard against which all other time zones are measured. UTC never changes for daylight saving and has no geographic location. It is the most reliable reference to use when communicating times across regions.

The difference between a local time zone and UTC, expressed as UTC+ or UTC− followed by hours and optionally minutes (e.g. UTC−5, UTC+5:30). Offsets describe the current difference only — they do not capture future changes such as daylight saving transitions.