from a NASA FAQ site:

Question: In most pictures of the astronauts, I have noticed that they wear two wristwatches. My question is: Why?

Answer: Actually, I'm wearing three things right now that look like wristwatches. We have three units of time up here, at least while we're docked with the space station. One is called Mission Elapsed time -- MET -- and that starts at zero when the shuttle lifts off. Everything on the shuttle is referenced to Mission Elapsed time.

We also have Greenwich Mean time because that's the time that the space station works on. That's the standard Greenwich Mean time that starts at zero on January 1 and counts up in days, minutes and seconds. The last time we have is Houston time, mainly because we want to know when our family is waking up and having dinner and going to sleep just so we can kind of keep in sync with them.

Wolfgang