[kbutler]

If it were just

11am 12am 1pm

That would make sense.

But it would be

11:59:59.9999999999am

12:00:00.0000000000am

12:00:00.0000000001pm

So incrementing the arbitrarily least significant unit would require toggling the most significant unit with no other digits changing.

And if you ever saw a clock displaying 12am, it should actually be p.m. by the time the light reached your eye.

Makes more sense to increment the a to p at the same time you roll over all those 99s and 59s to 0, then 12 is consistent however precise your clock is.