What Do AM and PM Stand For?

Two 12-Hour Periods

The 12-hour system divides the 24 hours of a day into two periods lasting 12 hours each.

Noon serves as the dividing point between the two periods and as the basis for the designations am and pm:

1. AM means before noon. This is the 12-hour period from midnight to noon.
2. PM means after noon. It covers the 12 hours from noon to midnight.

Using numbers from 1 to 12, followed by am or pm, the 12-hour clock system identifies all 24 hours of the day.

For example, 1 am is one hour after midnight, and 5 am is early in the morning; 5 pm is late in the afternoon, while 11 pm is one hour before midnight.

See an overview of all hours of the day

Midnight and Noon: AM or PM?

The main weakness of the 12-hour system is widespread confusion about which abbreviation should be used for noon and midnight. Neither moment can logically be identified as before noon (am) or after noon (pm).

For example, the moment of midnight occurs precisely 12 hours after noon on the previous day and 12 hours before noon on the following day.

However, most digital clocks and a majority of sources, including timeanddate.com, designate midnight as 12 am and noon as 12 pm. Although the precise moment of noon falls in neither category, the hour succeeding it, from 12:00:01 to 12:59:59, is clearly after noon (pm).

To avoid confusion when referring to the precise moment of noon or midnight, we recommend using the designations 12 noon and 12 midnight instead.

Tips for better teamwork across time zones

Midnight Confusion: Which Date Is It?

Another source of confusion is the lack of a date designator in the 12-hour system, which makes it impossible to logically identify a correct moment in time when only a date and 12:00 am (midnight) are provided.

Imagine being asked to pick up a friend at the airport at 12:00 am on April 13. Would you go there at midnight between April 12 and April 13? Or 24 hours later?

One way to overcome this problem is to sacrifice accuracy for clarity. Your friend could ask you to be at the airport at 12:01 am on April 13 or, if the following midnight is meant, at 11:59 pm on April 13.

Alternatively, the 24-hour format, sometimes referred to as military time, could be used. Here, 0:00 refers to midnight at the beginning of the day while 24:00 is midnight at the end of the day.

Conversion chart: military time, 12-hour format, and 24-hour format

Converting Times

A 24-hour clock states the time according to the number of hours that have passed since midnight.

Starting at midnight, hours are numbered from 0 to 24, removing the need for designations like am and pm.

For example, at 23:00, 23 hours have passed since the beginning of the current day. It corresponds to 11:00 pm in the 12-hour system.

Converting from 12-Hour to 24-Hour

• From midnight to 12:59 am, subtract 12 hours.
  12:49 am = 0:49 (12:49 – 12)

• From 1 am to noon, just remove the am.
  11:49 am = 11:49

• From 12:01 pm to 12:59 pm, just remove the pm.
  12:49 pm = 12:49

• From 1:00 pm to midnight, add 12 hours.
  1:49 pm = 13:49 (1:49 + 12)

Converting from 24-Hour to 12-Hour

• From 0:00 (midnight) to 0:59, add 12 hours and add am.
  0:49 = 12:49 am (0:49 + 12)

• From 1:00 to 11:59, just add am.
  11:49 = 11:49 am

• From 12:00 to 12:59, just add pm.
  12:49 = 12:49 pm

• From 13:00 to 0:00, subtract 12 hours and add pm.
  13:49 = 1:49 pm (13:49 - 12)

Where Is the 12-Hour Format Used?

Most countries around the world today use the 24-hour system.

However, the 12-hour format, including am and pm, is officially used in a number of countries, including the United States, Canada (except Québec), Australia, New Zealand, and the Philippines.

Why Does the Day Have 24 Hours?

The division of the day into 24 hours is often associated with ancient Egyptian timekeeping practices.

Based on historical and archaeological evidence, the Egyptians divided the day into 12 hours of daylight and 12 hours of night, using tools like sundials, shadow clocks, and water clocks. Night hours were tracked using the movement of groups of stars called ‘decans.’

While there is no single definitive source explaining the origin of the 24-hour system, the Egyptians’ innovations in timekeeping laid the groundwork for later practices.

Topics: Timekeeping, Clocks