Well, the T can be omitted if reading and writing parties agree: »NOTE: By mutual agreement of the partners in information interchange, the character [T] may be

Well, the T can be omitted if reading and writing parties agree: »NOTE: By mutual agreement of the partners in information interchange, the character [T] may be omitted in applications where there is no risk of confusing a date and time of day representation with others defined in this International Standard.« And I think SpaceX wouldn't even have the obvious option of using their own time zone. AFAIK mission control and launch are in different time zones anyway, so UTC is really the only sane option to make sure everyone refers to the same time.