Unless you're a surveyor or doing astronomical observations, for all practical purposes UTC, Greenwich Mean Time, and Zulu Time are the same.
Irony is lost on some people. Actually UTC and ZULU are exactly the same thing. Converting from UTC to ZULU is like converting from Hertz to Cycles per Second. The U. S. Naval Observatory denigrades the term GMT. It is imprecisely and ambiguously defined. So unless you're a news reader for the Beeb, you shouldn't use the term < affected snotty Oxbridge accent > Greenwich Mean Time. < /affected snotty Oxbridge accent >