12 "post meridian" is 12 hours after noon. 12pm. Which is midnight. But you don't use 12pm to mean midnight. Why not? Midnight doesn't actually belong to a day

12 "post meridian" is 12 hours after noon. 12pm. Which is midnight. But you don't use 12pm to mean midnight. Why not? Midnight doesn't actually belong to a day at all. Our current conventions of 12am and 12pm seem to be mostly products of digital time keeping.