You can use first principles. "12:00pm" likely refers to the minute that contains the seconds "12:00:00pm, 12:00:01pm, 12:00:02pm... 12:00:59pm". Each of the se

You can use first principles. "12:00pm" likely refers to the minute that contains the seconds "12:00:00pm, 12:00:01pm, 12:00:02pm... 12:00:59pm". Each of the seconds that follows the first one is non-ambiguous (they're in the afternoon). It would make little sense to have the first time be 12 hours apart from the other ones. Thus the boundary point 12:00pm refers to noon and the boundary point 12:00am refers to midnight.