It's not as good idea as you may thing:
Okay, it's "1700Z" and you're at work: are the folks at the Singapore office in yet? If New York's business hours are 14-22Z, what are Berlin's? Bangalore's? Tokyo's? Seattle's?
At least with timezones you can do some simple +/- math and see if you end up in the typical '9-5L' range:
But with only-UTC (as much I set my servers to it), you still have to keep a table of business hours.