I've always seen "UTC" expanded as Coordinated Universal Time. In addition, both the Wikipedia and Encyclopædia Britannica entries, as well as pretty much every reference to it I've ever seen that I can remember, spells it as such.
However, I often write diphthongs with diæreses. And, although for most if not all practical purposes I think most people would understand quite clearly what I mean if I write it as Coördinated Universal Time, from a technical standpoint is it the same thing?
After all, Coordinated Universal Time is a proper noun, which means it has a more rigid linguistic identity than common nouns, which can have homonymic alternatives; proper nouns often are less likely to have as such.