What is Coordinated Universal Time (UTC)?

Coordinated Universal Time, also known as UTC or ZULU time, is the standard on which civil time is based. It is also used for defining the individual time zones.

The aviation industry uses Coordinated Universal Time because it is the same in all of the world’s time zones, thus eliminating the need to do any time conversions when aircraft crosses time zones during flights. All time references within the aviation industry are indicated in relation to UTC, not the local time of the place where the time references originated.

Coordinated Universal Time is not adjusted for daylight saving time, and thus does not change in any manner. During the period of daylight saving time, UTC differs from the local time in London by 1 hour; in winter they are identical (the local time in London shifts by one hour while UTC remains the same = they become identical).

Mechanism and precision

UTC is determined using two components:

  • International Atomic Time (TAI) – a time defined based on the data from more than 200 atomic clocks.
  • Universal Time (UT, sometimes also UT1) – a time standard based on the Earth’s rotation.

UTC has succeeded GMT (Greenwich Mean Time) which took into account the slowing of the Earth’s rotation – this is not possible with atomic clocks. To make sure the Coordinated Universal Time can be used in real life, it is kept within ±0.9 seconds of the Universal Time (UT1). When the permitted interval is exceeded, a leap second is removed (or inserted) on midnight 30 June or 31 December (whichever occurs sooner). Such a day therefore ends on 11.59:60 PM or 11.59:58 PM, as the case may be.

Time zones

There are 40 time zones in total, with their size depending on the country borders. The basic time zone is referred to as UTC. Local time is then determined by adding (or subtracting) a specified number of hours – for example UTC+1 (time in Spain).


Universal Time (UT) was established in 1884. UTC was developed based on UT in 1960, officially replacing GMT seven years later (in 1967). In 1972 the leap-second system was introduced, finally stabilising Coordinated Universal Time for good.