GMTUTC

GMT and UTC are two related time standards commonly referred to together as GMT/UTC. Greenwich Mean Time (GMT) is a historic civil time used as a reference for the Prime Meridian at 0° longitude, originally based on the mean solar time at Greenwich. It has long served as a time zone in the United Kingdom and some other regions, and in casual use “GMT” is often used to denote the time at 0° longitude.

Coordinated Universal Time (UTC) is the modern international time standard used for most civil, scientific, and technical purposes. UTC is not a time zone per se but a time scale maintained by a network of atomic clocks. It is kept within about 0.9 seconds of UT1, a time scale based on Earth's rotation, by occasionally inserting leap seconds. This adjustment ensures UTC remains in close agreement with solar time while supporting precise, globally synchronized timing.

In practice, UTC serves as the reference from which other time zones are offset (for example, UTC+02:00). While GMT and UTC share the same current offset of zero hours, GMT is historically tied to a geographic meridian and a time zone, whereas UTC is the precise, continuous time standard used in aviation, computing, telecommunications, and international timekeeping.

Although interchangeable in everyday language, the authoritative term in modern contexts is UTC for the time standard and GMT for historical or regional usage.