Universal time and legal time in Belgium

The Earth rotates in 24 hours. The solar time is therefore permanently different from one place to another. Until 1 May 1892, Belgium used local (mean) time, which was most often that of a large nearby city. Towards the end of the 19th century, the Earth was divided into 24 time zones, and the law of 28 April 1892 (which is implemented on 1 May 1892) unified time for the whole country. According to this law, the reference time was the mean solar time of the Greenwich meridian, called GMT (Greenwich Mean Time).

In 1971, the official world reference time became the Universal Coordinated Time (UTC). This time is based on the atomic time, defined in 1967 from a quantum property of the caesium atom. In this system, the time zone UTC + 0 h corresponds to that of the Greenwich meridian. Since 10 September 2018, legal time in Belgium has been aligned with UTC.

The old GMT name is still sometimes used instead of UTC. However, the two terms are not equivalent. Indeed, until 1925, according to GMT, the day began at noon and not at midnight. In addition, GMT is based on astronomical criteria (the rotation of the Earth), while UTC is based on the atomic time. Indeed, the Earth does not always turn exactly around itself in 24 hours since its rotation is disrupted by various physical processes. The atomic time, on the other hand, is much more stable. In order to keep UTC as close as possible to the Earth rotation, leap seconds are added at certain times.

The Royal Observatory of Belgium realises the UTC for the whole country. For more information on UTC, the definition of the second and leap seconds and the work of the Royal Observatory of Belgium on time, visit the website of the Time Office.