...so I guess I have to start a new thread to make a small suggestion, which is that you call it UTC ("Coordinated Universal Time) instead - or in addition - to calling it GMT, which is now archaic. GMT still means something in the UK. The UTC system was launched in 1961 and it replaced GMT as the official (and universal standard) for global time in 1972, 36 years ago. And since Zulu time (which also works out to be more or less the same, essentially, in practice, as GMT) is a military system, and since we all believe in making love, not war....
Standard format for UTC time is that of a 24 hour clock (no am or pm) plus the date, with local times expressed as an offset from UTC (plus or minus) however many hours, plus an additional offset for daylight savings times when needed).
Although UTC time is almost the same as GMT (mean solar time at Greenwich, England, also known as UT1), the two are not identical. UTC is defined by reference to atomic clocks and the like, not the position of the sun relative to an observatory near Bath). Since UTC incorporates occasional leap seconds to accommodate the slowing rate of rotation of the earth, UTC and GMT/UT1 can depart from each other by up to 0.9 seconds.
This is hardly irrelevant to diving, since the deceleration of the rotation of the earth is caused by the friction of tidal currents! Since the cross-over to UTC in 1972, there have been 23 leap seconds added to keep UTC in synch with solar time, an average of one every 18 months. (You may have thought that days were getting shorter, but actually the days are getting longer).
Your dive computer, however, probably doesn't care.
- Frogfish2 (leading the fight against temporal archaism on your local dive forum!)