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.
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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.