AM and PM — Definition, Formula & Examples
AM and PM are the two labels used in the 12-hour clock system to tell the first half of the day apart from the second half. AM covers midnight (12:00) through 11:59 in the morning, while PM covers noon (12:00) through 11:59 at night.
AM (ante meridiem) and PM (post meridiem) are Latin abbreviations that partition a 24-hour day into two consecutive 12-hour periods. Ante meridiem designates the interval from 12:00:00 midnight to 11:59:59, and post meridiem designates the interval from 12:00:00 noon to 23:59:59 in equivalent 24-hour notation.
How It Works
When you read a 12-hour clock, the number alone does not tell you whether it is morning or evening — that is the job of AM or PM. Times from 12:00 midnight up to 11:59 in the morning are labeled AM. Starting at 12:00 noon and continuing through 11:59 at night, times are labeled PM. To convert a PM time (other than 12 PM) to 24-hour format, add 12 to the hour. To convert an AM time (other than 12 AM), keep the hour as-is. The tricky cases are 12:00 AM, which equals midnight (0:00 in 24-hour time), and 12:00 PM, which equals noon (12:00 in 24-hour time).
Worked Example
Problem: A movie starts at 7:45 PM and runs for 2 hours and 30 minutes. What time does it end, in 12-hour format with AM or PM?
Start time: The movie begins at 7:45 PM.
Add the hours: Add 2 hours to 7:45 PM to get 9:45 PM.
Add the minutes: Add 30 minutes to 9:45 PM. Since 45 + 30 = 75 minutes, that is 1 hour and 15 minutes. Carry the extra hour forward.
Check AM or PM: 10:15 is still before midnight, so it stays PM.
Answer: The movie ends at 10:15 PM.
Another Example
Problem: Convert 3:20 PM and 12:00 AM to 24-hour (military) time.
Convert 3:20 PM: Because the time is PM and the hour is not 12, add 12 to the hour.
Convert 12:00 AM: 12:00 AM is midnight. In 24-hour time, midnight is written as 0:00 (or 00:00).
Answer: 3:20 PM = 15:20 and 12:00 AM = 0:00 in 24-hour time.
Why It Matters
Understanding AM and PM is essential any time you read a clock, set an alarm, or plan a schedule. In elementary and middle-school math courses, time-word problems frequently require converting between 12-hour and 24-hour formats. Professions such as healthcare, aviation, and broadcasting rely on correct AM/PM usage to avoid dangerous scheduling errors.
Common Mistakes
Mistake: Saying 12:00 AM is noon and 12:00 PM is midnight.
Correction: It is the opposite: 12:00 AM is midnight (the start of a new day), and 12:00 PM is noon. Remember that AM hours come first in the day.
Mistake: Adding 12 to 12 PM when converting to 24-hour time, getting 24:00.
Correction: 12 PM already equals 12:00 in 24-hour time. Only add 12 to PM hours that are 1 through 11.