That all made sense except for "is that 12pm noon or midnight?" 12pm is always noon, 12am is always midnight, and anyone who suggests otherwise is wrong.
Not according to the NIST, which simply lists 12pm and 12am as "
ambiguous". Wrong or otherwise, I have frequently encountered people who use 12pm to mean midnight. IBM used to recommend, and might still do for all I know, that events should
never be scheduled for noon or midnight to avoid these problems of ambiguous usage.
Using 12am for midnight makes no sense, since it suggests that the first hour of a day is actually the
last hour of the day before. Neither does making
midnight in the twenty-four hour clock 24:00, another strange habit I have encountered, which is apparently justified on the odd idea that there are two midnights in a day, something that could easily lead to the temporal equivalent of
fencepost errors.
GMT is
ambiguous, obsolete in technical contexts, and not synonymous with UTC, as GM points out.
