The abbreviations a.m. and p.m. help us specify which half of the day we're talking about. a.m. covers the hours from midnight to noon (the morning and late night hours), while p.m. covers noon to midnight (the afternoon and evening hours). They're written in uppercase or lowercase, and sometimes without the periods, like “4 p.m.” or “7 AM.”

These abbreviations come from Latin phrases: ante meridiem (before midday) and post meridiem (after midday). Without them, saying “I'll meet you at 7:00” creates confusion: do you mean morning or evening? With them, “7:00 a.m.” clearly means early morning, while “7:00 p.m.” means early evening.

The system can seem odd at first. Midnight is 12:00 a.m. (the start of the a.m. period), and noon is 12:00 p.m. (the start of the p.m. period). A minute after midnight becomes 12:01 a.m., and a minute after noon becomes 12:01 p.m. Many people find the 12:00 transitions confusing, which is why some prefer saying “noon” or “midnight” instead of using a.m. or p.m. at exactly 12:00.

Much of the world uses a 24-hour clock instead, counting from 00:00 to 23:59, which eliminates this confusion entirely. But in the United States, a.m. and p.m. remain the standard way to tell time.