Full Form of AM and PM | What is Ante Meridiem and Post Meridiem


This write-up will take you through the concepts and usage of the abbreviations “a.m.” and “p.m.” Also, find a full form of AM and PM.

After going through this write-up you will develop a better knowledge base of the full forms, meaning, origin, writing styles, and more.

Referring to time is the daily part of our lives. You regularly use or encounter the abbreviations “a.m.” and “p.m.” However, do you know what they mean? What is their origin?

Also, explore the 12-hour and 24-hour concepts and the midnight confusion.

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What are the full forms of a.m. and p.m.?

The full forms of a.m. and p.m. are as follows:-

  • AM – Ante Meridiem
  • PM – Post Meridiem

What is a.m. and p.m.?

Ante Meridiem refers to the time before mid-day. This time convention is used with the 12-hour clock and divides the time of a day into two equal periods. “a.m.” illustrates the time spanning from midnight to noon.

Post Meridiem refers to the time which begins after the midday. It is the 2nd convention alongside “a.m.” that is used with the 12-hour clock. “p.m.” portrays the 2nd period of the days that begin from noon to midnight.

The terms “a.m.” and “p.m.” are derived from the Latin words. They divide the day into two equal periods of 12 hours each.

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What is the writing style of a.m. and p.m.?

For formal invitations, several styles are adopted and acceptable. However, for print media and electronic communications, there is a specified style to use.

The number of times should be used instead of noon and midnight.

Also, to separate the hours and minutes, a colon is used and not ciphers (double zeros) with whole hours.


  • Write in lowercase a.m. and p.m.
  • Always use periods.
  • Write noon and midnight in lowercase
  • Use only noon or midnight and not ‘12 noon’ or ‘12 midnight’.
  • With a.m. or p.m., do not use o’clock.
  • Use dashes () rather than “to” or “and” or “through” or “until”.

What is the midnight confusion?

Standing as a source of confusion is the lack of a date designator in the 12-hour clock system.

This makes it impossible to identify the appropriate moment in case the only date and 12:00 am being provided.

For Example

You have an important work due at 12:00 am on May 25. Would you do it at midnight between May 24 and May 25? Or would you do it 24 hours later?

This problem can be tackled by sacrificing accuracy to attain clarity. This means that either you can submit the assignment at 12:01 a.m. on May 25 or at 11:59 p.m. on May 25.

This depends on what you have adopted as an acceptable measure.

Several airlines adopt the system using 12:01 a.m. to mark the beginning of the day.

They mark the end of the day at 11:59 p.m. This ensures clarity rather than accuracy.

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How to convert 12-Hour to 24-Hour format?

The 24-hour clock is sometimes referred to as military time and States the time as per the number of hours that have passed since midnight.

The 24 hours of the day are numbered from 0 to 24. This removes the need to use designations like a.m. and p.m.

The following rules are used to convert a.m. or p.m. time to the 24-hour format used:

Subtract 12 from the given time

For Example

To convert 13:25, just deduct 12 from the hour portion. The given time will be 1 in the 12-hour format.

  • Put the minute portion back. This will make it 1:25.
  • Add the designation of p.m. Hence, you have the time 1:25 p.m. in 12-hour format.

Where is the 12-Hour format used?

Most of the countries in the world today use the 24-hour system.

Several countries use the 12-hour time format as well such as Canada (except Quebec), the United States, and Australia, the Philippines, New Zealand, and more.

Why does the day have 24-Hours?

The time system of 24 hours in a day with every hour divided into 60 minutes in every minute divided into 60 seconds is interesting as well as complex.

The 24-hour day is derived from the ancient Egyptians who divided the daytime into 10 hours which was measured with several devices.

They are added twilight hour at the beginning and another one at the end of the day-time.

They divided the night time into 12 hours which was based on the observation of stars.

The Egyptians had ‘decans’, a system of 36-star groups, which were chosen so that any night one decan rose 40 minutes after the previous one.

Deceptions produced tables that helped people to determine the time at night by observing the stars.

However, the length of the day and night are unequal and varied with the seasons and observations.

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