12 hour clock
The 12 hour clock uses a.m. to write the time between midnight and midday. For example, 8:43 a.m. Therefore, p.m. is used to write the time between midday and midnight. For example, 8:21 p.m.
The time at midnight is 12:00 a.m.
The time at midday is 12:00 p.m.