What is UTC?

Greenwich Mean Time (GMT) is not the world’s time standard.

Coordinated Universal Time (UTC) is the world’s 24 hour standard for civil time. UTC is determined by combining the Earth’s rotation (UT1) and compares it to the exact length of a day based on Atomic Clock Time (TAI).

In 1960, the concept of UTC was formalized. It was determined by the International Radio Consultative Committee in 1967 that the world should adopt “Coordinated Universal Time” in order to closely synchronize their time scales. In 1972, the concept of “leap seconds" was introduced to eliminate the need to adjust UTC.

The abbreviation UTC (not CUT or in French, TUC) was decided upon by the International Telecommunication Union (ITU) and the International Astronomical Union. For consistency, choosing UTC aligned to existing acronyms for Universal Time, which is UT, UT0 and UT1, designating one single acronym for use in all languages. UTC, therefore, is language agnostic.

Greenwich ceased to become a “Transit Circle” and became a time zone, but remains the Prime Meridian original reference at 0° longitude.