What Is UTC?
Coordinated Universal Time (UTC) is the primary time standard by which the world regulates clocks and time. It is maintained by a worldwide network of atomic clocks and is the successor to Greenwich Mean Time (GMT).
UTC vs. GMT: What's the Difference?
While UTC and GMT are often used interchangeably in everyday contexts, they are technically different. GMT is based on the Earth's rotation relative to the sun — an astronomical measurement. UTC is based on atomic time, with occasional adjustments (leap seconds) to keep it within 0.9 seconds of GMT.
For all practical purposes, UTC and GMT are the same. The difference only matters in highly precise scientific applications.
How Leap Seconds Work
Earth's rotation is not perfectly constant — it gradually slows due to tidal friction from the Moon. To prevent UTC from drifting too far from solar time, the International Earth Rotation and Reference Systems Service (IERS) occasionally adds a "leap second" to UTC.
Leap seconds are added (or in theory, subtracted) at the end of June 30 or December 31. Since their introduction in 1972, 27 leap seconds have been added. However, leap seconds cause significant problems for computer systems, and the ITU has proposed abolishing them by 2035.
Why Developers Should Use UTC
Always store timestamps in UTC. Convert to local time only for display. This avoids countless bugs caused by DST transitions, timezone ambiguity, and timezone offsets. The Unix timestamp (seconds since January 1, 1970, 00:00:00 UTC) is the most unambiguous way to store a moment in time.
Sources
- IERS: International Earth Rotation and Reference Systems Service (iers.org)
- BIPM: Bureau International des Poids et Mesures (bipm.org)
- ITU-R TF.460: Standard Frequencies and Time Signals