UTC, yes, but that doesn't resolve the problem of establishing office hours. At least when you do explain those you don't need to constantly convert.

"Here we work from 17h to 23h" is a lot easier to understand "Here we work from 9 to 5, but we're in summer time, so that's actually an hour off of the usual thing but...oh, wait, you're on summer time now too, so I guess it's the usual plus six hours, no wait, minus six..."

One planet, one time-zone, and everyone can keep their local customs for waking/sleeping, working, and whatever else.