UTC and GMT Explained — What Is the Difference?

UTC vs GMT: The Definitive Guide

UTC (Coordinated Universal Time) and GMT (Greenwich Mean Time) are often used interchangeably, but they are technically different.

Greenwich Mean Time (GMT)

GMT is a timezone based on the mean solar time at the Royal Observatory in Greenwich, London. It was established in 1884 at the International Meridian Conference as the world's time standard. GMT is defined by the rotation of the Earth relative to the Sun.

Coordinated Universal Time (UTC)

UTC is a time standard (not a timezone) maintained by the International Bureau of Weights and Measures (BIPM). It is based on International Atomic Time (TAI) with leap seconds added to keep it within 0.9 seconds of astronomical time (UT1).

Key Differences

Aspect	GMT	UTC
Type	Timezone	Time standard
Basis	Earth's rotation	Atomic clocks
Precision	Varies with Earth's rotation	Constant (with leap seconds)
Official use	UK winter time	International standard

The Name "UTC"

The abbreviation "UTC" is a compromise between the English "Coordinated Universal Time" (CUT) and the French "Temps Universel Coordonné" (TUC). Neither language's abbreviation was chosen to maintain neutrality.

Practical Difference

For most practical purposes, UTC and GMT are identical — both represent the time at 0 degrees longitude with no daylight saving adjustment. However:

• Use UTC in software, APIs, databases, and logging
• Use GMT only when referring to the UK timezone during winter (which is Europe/London in the IANA database)

UTC in Programming

// JavaScript — always use UTC for storage and computation
const now = new Date();
console.log(now.toISOString());     // "2024-03-15T14:30:00.000Z"
console.log(now.getTime());          // 1710513000000 (ms since epoch)

// The "Z" in ISO 8601 stands for "Zulu time" which equals UTC

Why Epoch Is Based on UTC

The Unix epoch (January 1, 1970 00:00:00) is defined in UTC. This means Date.now() in JavaScript always returns milliseconds since the UTC epoch, regardless of the user's local timezone.