EOY

UTC, GMT and leap seconds

Every time zone is an offset from one reference: Coordinated Universal Time. UTC is what atomic clocks say, nudged now and then to stay in step with the turning Earth.

What UTC is

UTC is the primary time standard used worldwide to regulate clocks; it is the basis for civil time and for every time zone's offset. It does not change with the seasons — daylight saving happens in local time, never in UTC. When the board says a zone is UTC+5:30, it means clocks there read five and a half hours ahead of this reference.

Atomic time, and a correction for the Earth

UTC is derived from International Atomic Time, a weighted average of hundreds of atomic clocks around the world. The Earth's rotation is not that regular: it varies with climatic and geological events, and on average it is slowing very slightly. To keep UTC within 0.9 seconds of the time defined by the Earth's rotation, a leap second is occasionally inserted. Since the system began in 1972, 27 leap seconds have been added, the most recent on 31 December 2016 — so on some New Year's Eves the last minute of the year has had 61 seconds.

GMT versus UTC

Greenwich Mean Time is the older reference, named for the Royal Observatory's meridian chosen at the 1884 conference. In everyday use the two are treated as the same thing, and UTC is the effective successor to GMT; strictly, GMT is the older designation for the zero-offset time zone and UTC is the standard that zones are measured from. The board labels zero offset simply as UTC.

The end of leap seconds

Leap seconds are irregular and unpredictable, and software handles them badly — after the leap second of 30 June 2012, sites including Reddit and Mozilla and many Linux systems reported failures. In November 2022 the General Conference on Weights and Measures resolved to increase the permitted gap between atomic time and Earth time "in, or before, 2035", which ends leap seconds as we know them. Clocks will drift a minute from the Sun over a century or so; a later generation will decide what to do about it.

What this means for the countdown

The board reads your device's clock, which is kept in step with UTC by your operating system, and computes the exact time to midnight in the selected zone. If a leap second were ever added again on 31 December, the board would follow the clock, exactly as it follows it across a daylight saving change.

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