UTC
acronymprogrammingnetworking
Stands for: Coordinated Universal Time
The primary civil time standard the world sets clocks by.
Coordinated Universal Time is the basis for civil time worldwide. It tracks atomic time (TAI) but inserts occasional leap seconds to stay within a second of solar time, and time-zone offsets are expressed relative to it.
UTC is the time standard everything technical should log in, and the discipline it enforces is that a timestamp without a zone is not a timestamp. Correlating events across systems in different regions is impossible if each writes local time, and daylight saving makes local time non-monotonic: an hour repeats every autumn, so two genuinely different events can carry identical local stamps.
It is worth separating from the related terms it gets confused with. GMT is a time zone; UTC is a standard. Unix time counts seconds since the 1970 epoch and is zone-free by construction, which is why it is the safest thing to store. ISO 8601 with an explicit offset is the safest thing to display, because it is unambiguous to both humans and parsers.
The complication most systems ignore until it bites is the leap second, inserted irregularly to keep atomic time aligned with the earth's rotation, which has caused real outages when software assumed every minute has sixty seconds. The practical rules are simple: store UTC, display local, never compute across zones without a real library, and keep clocks synchronized, because skewed clocks make logs disagree about what happened first.
Also known as: utc