What time zone is used on ISS to define when a day starts? Is it the same as mission control? Do Russian cosmonauts use a different time zone respect to European and American astronauts?

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    $\begingroup$ Reading again question, I think that needs clarification. Ok, they use UTC, but do they wake up at 7.00 AM UTC? I do not know if accept answer or edit question and wait for edit on answers. $\endgroup$
    – user55
    Jul 18, 2013 at 9:10
  • $\begingroup$ I would ask another question, as there are already good answer giving what seemed to be requested, and fix this question to be what everyone thought it should be. I would ask something like "What time does a day start/end on the ISS, and is it related to an Earth time zone?" $\endgroup$
    – PearsonArtPhoto
    Jul 19, 2013 at 13:30
  • $\begingroup$ @PearsonArtPhoto: You suggested splitting off another question; I have done so: space.stackexchange.com/q/31317/26446 $\endgroup$
    – DrSheldon
    Oct 13, 2018 at 15:54

4 Answers 4


Virtually all spacecraft use one of two time conventions, or more likely, both of these

  1. Universal Time Coordination (UTC)
  2. GPS Time

The difference between these two is slight, but changes periodically. The difference is currently 16 seconds, as can be seen on this clock. The reason for the difference is the failure for GPS time to keep track of leap seconds (This is deliberate, to not have to worry about missing seconds). The reason for this is that it is an international standard, established for things where a common time metric is needed across the world. Satellites move through 24 time zones in a very short period of time, the ISS less than 100 minutes, so it makes sense to have a common anchor point. Typically, navigation systems use GPS time, while other systems use UTC time, and that is exactly what is used on the spacecraft.

Typically, the spacecraft time is set to UTC even for non-Earth orbiting systems, where the speed of light delay is significant.


Like many armed forces, and Stack Exchange, they use UTC (or Zulu time)

Historically, Greenwich Mean Time was accepted as being the definitive time zone in 1884 to aid navigation (which then, as now, relied on accurate time) - and the Prime Meridian was defined as running through Greenwich at 0 degrees Longitude.

Coordinated Universal Time (UTC) succeeded Greenwich Mean Time in 1963 and is basically GMT with better (sub-second) accuracy.


While it is true that just about everything uses UTC on the ISS, the crew's schedule moves around. For example, if there is an upcoming EVA that must be done at a particular time, or a spacecraft arrival/departure scheduled, ground control will adjust the sleep cycle of the crew over several days. Thus the "day" as defined on the station by when the crew is awake can and does change.


UTC. It is kind of middle ground between mission controls in Houston and Moscow.

  • $\begingroup$ Also, it's internet time. $\endgroup$
    – user12
    Jul 17, 2013 at 21:39
  • $\begingroup$ There is a nice statement by Chris Hadfield of why UTC is used (besides military, Antarctic crews, the internet etc.) - I just can not find it. If someone does, feel free to edit the link into the answer. $\endgroup$
    – s-m-e
    Jul 17, 2013 at 21:42
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    $\begingroup$ Antarctic crews don't use UTC, if close to the south pole they use whatever the nearest non-Antarctic supply station uses, and if close to the coast they use what fits best with local time. $\endgroup$
    – gerrit
    Jul 17, 2013 at 22:15
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    $\begingroup$ You may be right, but when mailing with them, UTC is the time zone of choice for handling appointments, describing data etc. ... And I very much do not care about when they sleep ;-) $\endgroup$
    – s-m-e
    Jul 17, 2013 at 22:20
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    $\begingroup$ spaceflight.nasa.gov/station/crew/exp7/luletters/… - "First off, we live on Greenwich Mean Time (GMT) -- which is a time zone roughly halfway between Houston and Moscow, where our two main control centers are located. So when we wake up (at around 7:00 GMT), it is 2 in the morning in Houston and 11 in the morning in Moscow. " $\endgroup$
    – osgx
    Mar 23, 2014 at 14:09

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