Time - Greenwich Mean Time (GMT | UT)

Data System Architecture


Universal Time (UT) is a time standard based on Earth's rotation on the Prime Meridian at Greenwich and is therefore also known as the Greenwich Mean Time (GMT)

  • Greenwich Mean Time (GMT) is the civil name
  • UT is the scientific name

You may encounter it with the name wiki/mean solar time)

The UT1 time-scale captures the accurate length of day, but is only available some time after the day has completed.

GMT is a universal standard which does not make any seasonal adjustments (i.e. does not observe Daylight Savings Time).

When it was midnight on 1970-01-01 in China, it was 4pm on 1969-12-31 in Greenwich.

Difference with UTC

The distinction between UTC and UT is that:

  • UTC is based on an atomic clock
  • and UT is based on astronomical observations

Because the earth's rotation is not uniform (it slows down and speeds up in complicated ways), UT does not always flow uniformly.

Leap seconds are introduced as needed into UTC so as to keep UTC within 0.9 seconds of UT1, which is a version of UT with certain corrections applied.


  • php: gmdate returns a time in Greenwich Mean Time (GMT)

Documentation / Reference

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