2. Time, what Time?

Last update: April 3, 2024 18:24 UTC (fe4830d85)

This section discusses general topics related to time and time-keeping.

2.1 What is Time?

This quote from a fortune cookie from Edition 7: “Time has been invented in the universe so that everything would not happen at once.”

There’s another interesting relation between time and change: There is no change without the concept of time, and there is no movement without time.

Philippe Eveque contributed:

Nobody can tell what Time is. However everybody does know its effects. Time implies memory and this is specially strange to consider this, in the context of computer science. Reversely does memory implies time in some way?

2.2 What is UTC?

UTC (Universal Time Coordinated, Temps Universel Coordonné) is an official standard for the current time. GMT (Greenwich Mean Time) was originally used to set the clocks on ships before they left for a long journey; it was eventually adopted as the world’s standard time. UTC evolved from GMT.

One of the reasons for replacing GMT as official standard time was the fact that GMT was based on mean solar time. Newer methods of time measurement show that the mean solar time varies.


The UTC second was defined by the 13th General Conference of Weights and Measures in 1967 as: “The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom.”

2.3 Is UTC the same as GMT?

The observatory in Greenwich derived GMT from astronomical events like the solar day. UTC is based on a quantum resonance of a cesium atom, being quite more accurate.

Unfortunately the earth’s rotation is not very much impressed by the definition of the UTC second. Having 86400 UTC seconds per day on an earth that’s slowing down would mean that midnight would eventually fall in the middle of the day. As this is probably unacceptable, some extra seconds can be added or removed inside the UTC time-scale to keep synchronization. That patch work is named leap seconds.

To make things worse, leap seconds can be predicted as much as the earth’s rotation, which is not at all. Therefore you can’t easily make calculations for dates in the future using UTC; at least not with accuracy of a few seconds.

2.4 What happens during a Leap Second?

During a leap second, either one second is removed from the current day, or a second is added. In both cases this happens at the end of the UTC day. If a leap second is inserted, the time in UTC is specified as 23:59:60. In other words, it takes two seconds from 23:59:59 to 0:00:00 instead of one. If a leap second is deleted, time will jump from 23:59:58 to 0:00:00 in one second instead of two.