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CH3MTR4IL5 said.. Well if you're receiving a GPS signal you also have a time sync signal as part of the data packet. Why not just use that and run on earth time? Why need a standalone clock?
Just running this randomly through my brain, not sure how relevant the below is.
Computers need an internal clock, maybe it's something to do with that.
The GPS signal has to be adjusted to UTC from earth clocks, space clocks run on their own time.
Once you're on the far side of the moon you get no earth gps signal, so you need the moon's gps signal.
Maybe they have to agree on a time sync period, so all clocks adjust simultaneously.
Way over my head I can't give a definite answer.