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 th

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.