AM vs. PM: What time is it?

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Difference between A.M. and P.M.

For a simplified manner of dividing the 24 hours in a day, civilizations have adopted time conventions. According to the one almost everyone uses today, the first 12 hours of the day are referred to as ante meridiem hours (abbreviated as a.m.) and the later 12 hours are post meridiem hours (abbreviated as p.m.). For instance, the military time convention of 15:00 becomes 3 p.m.; 22:00 becomes 10 p.m for the rest of us.

A discussion is still open related to the correct form for noon and for midnight. 12 p.m. and 12 a.m. as this can cause a confusion of perception. Many propose that it is helpful to use 12 midnight and 12 noon to clearly distinguish the two time references.



The division of the 24-hour day into 2 halves of 12 hours each can be followed down through history back to ancient Egypt and Mesopotamia (modern Iraq and Syrian, Iran and Turkey) territories. For daytime hours which we now refer to as ante meridian or a.m., Egyptians had a sundial in order to name the hour corresponding to the position of the sun. During the night, when such a mechanism didn't have a source of sunlight, they used a water clock for the other 12 hours considered post meridian. The sundial and the water clock devices were discovered in a pharaohs' tombs dating back to 1500 years before Christ.

Romans also used the 12-hour clock. For them, the day was made of 12 hours and night hours were divided into four watches. They used a special meaning for time conventions. For instance 4 a.m. meant 4 hours before noon, not 4 hours after midnight. This is due to the fact that Romans counted morning hours in reverse. In the 14th century, the 24 hours clock became more popular. But people started seeing the economic advantages of using the 12-hour clock again in the centuries that followed.

Variations in Use

The 12-hour clock with a.m. and p.m. time conventions was used in ancient times in Egypt, Mesopotamia and the Roman Empire. It became popular also in northern Europe in the 16th century. The same system was used in British colonies. Some cultures even preserve this system today in places India and New Zealand. The United States and the English speaking provinces of Canada have implemented this time convention which is more popular than the 24 hour clock which runs from 0:00 to 24:00 hours.

Technology Difficulties

Scientists still argue about the difficulties the a.m./p.m. system and what it is representing when it has to be incorporated in modern digital electronics especially when the alphabetical order doesn't match the chronological order.

Similarities and Differences

  • The a.m. and p.m. system originated in ancient Egypt and Mesopotamia. It was used by Romans and spread throughout Europe and the colonies.
  • The 12 hour clock is now used mostly in English speaking territories like the United States, Canada, Great Britain, India and New Zealand.
  • Some scientists consider the a.m. and p.m. system to be incompatible with digital electronics which are used on a large scale in modern society.


comments 1 Comments

  • arwyn jones . 3+ yrs. ago

The 12 hour clock is NOT in use in the UK (what you wrongly call 'Great Britain') in formal uses, for instance, transport timetables. Most TV schedules use the 24 clock. Digital clocks use the 24-hour system.

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