Apparently, there is no ISO norm for am/pm
I have found style guides calling for both "10am" and "10 a.m.";
I have found style guides calling for upper case, small caps and lower case;
I have found style guides saying 12 is neither am nor pm, so say "12 noon" or "12 midnight" instead;
I have found style guides saying do not use numbers at 12 - just say "noon" or "midnight";
I have found style guides saying don't even use "midnight", because it's ambiguous which day it's in.
Wherever the usage is computer-driven, 12.00pm will always mean noon; but informally, plenty of people will argue the opposite.
The 30th edition of the U.S. Government Style Manual (2008) sections 9.54 and 12.9b recommends the use of "12 a.m." for midnight and "12 p.m." for noon. The 29th edition recommended the opposite!