Air Traffic Services units shall use Coordinated Universal Time (UTC) and shall express the time in hours and minutes and, when required, seconds of the 24-hour day beginning at midnight.
Coordinated Universal Time (UTC) shall be used by all stations in the aeronautical telecommunication service.
The reference time Coordinated Universal Time or UTC can be called “Zulu” time

Air Traffic Services units shall be equipped with clocks indicating the time in hours, minutes and seconds, clearly visible from each operating position in the unit concerned.

This time is set using several time references in some science laboratories.

Midnight shall be designated as 2400 for the end of the day and 0000 for the beginning of the day.

A date-time group shall consist of six figures, the first two figures representing the date of the month and the last four figures the hours and minutes in UTC. (examples are found in METARs and TAFs).

The time origin is Greenwich Mean Time (GMT) which is set to the Greenwich meridian 0°.

Standard time

Depending on its position in the world, a country can adopt its own hour named "standard time".

Prior to the introduction of standard time, each municipality throughout the clock-using world set its official clock, if it had one, according to the local position of the Sun.

Standard time is the synchronization of clocks within a geographical area or region to a single time standard, rather than using solar time or a locally chosen meridian (longitude) to establish a local mean time standard.

These regions divide the world into twenty-four time zones, each one covering 15 degrees of longitude.

In conclusion, local country standard time will differ around the world.
Be aware that some countries adopt local hour shifts between spring and winter period in order to save power consumption.

Problems when using standard time

For a flight which takes off at 13H00 standard time at Göteborg-Landvetter airport ESGG (Sweden) and lands at 8H00 standard time at Hong-Kong VHHH (China), if you do not know the standard time reference, it is difficult to know the exact number of hours this flight takes.

The result is certainly not 19 hours (19H = (08H – 13H) + 24H)

UTC or “Zulu” Time

Universal Time (UT) is a time standard based on the Earth's rotation.

Coordinated Universal Time is the primary time standard by which the world regulates clocks and time.

Nearly all UTC days contain exactly 86,400 seconds.

UTC (Coordinated Universal Time) is an atomic timescale that approximates UT.
UTC is also the time standard used in aviation to avoid confusion about time zones and daylight saving time for
  • flight plans
  • air traffic control clearances
  • weather forecasts and maps

Now with the example given in the previous chapter, if you know that the flight takes off at 12H00 UTC on ESGG and lands at 24H00 UTC on VHHH, with the same reference, we are now sure that the flight takes 12Hours ( 12H = 24H – 12H)

That is why all aircraft flight plans and aerodromes adopt UTC time in order to have the same reference all around the world and do not take into account the local hours offset during their flight and also for calculation purposes.
UTC time is mandatory for all flights in order to have the same worldwide reference.
A time check has to be done before taxiing for take-off.

Standard time zone

Standard time zones around the world are expressed using positive or negative offsets from UTC, as in the list of time zones by UTC offset.

The difference between UTC time and the standard time is set by national regulation and the difference is given with a multiple of hours and sometimes one half hour is added.

The table below shows an example of offsets used to calculate the standard time in some countries:

City (country) standard time
London (UK) UTC+0
Paris (France) UTC+1
Ankara(Turkey) UTC+2
Manama(Bahrain) UTC+3
Teheran(Iran) UTC+3:30
Moscow(Russia) UTC+4
Karachi(Pakistan) UTC+5
Delhi(India) UTC+5:30
Yekaterinburg(Russia) UTC+6
Phnom Penh(Cambodia) UTC+7
Canton(China) UTC+8
Tokyo(Japan) UTC+9
Canberra(Australia) UTC+10
Nouméa(New Caledonia) UTC+11
Wellington(New Zealand) UTC+12
Nuku’olofa(Tonga) UTC+13
Pago Pago(Samoa) UTC-11
Tahiti(French Polynesia) UTC-10
Anchorage(USA) UTC-9
San Francisco(USA) UTC-8
El Paso(USA) UTC-7
Chicago(USA) UTC-6
New York(USA) UTC-5
Caracas(Venezuela) UTC-4
Halifax(Canada) UTC-4
Rio de Janeiro(Brazil) UTC-3
Fernando de Noronha(Brazil) UTC-2
Acores(Portugal) UTC-1

See also


  • ICAO Documentation Annex 10 - Aeronautical Telecommunications - Volume II - Communication Procedures - Chapter 3.4
  • ICAO Documentation Annex 11 - Air Traffic Services - 14th Edition - July 2016 - Chapter 2.26


  • VID 150259 - Creation - Wiki integration
  • VID 150259 - Update September 2019


  • 12:31, 11 January 2020


  • This documentation is copyrighted as part of the intellectual property of the International Virtual Aviation Organisation.


  • The content of this documentation is intended for aviation simulation only and must not be used for real aviation operations.