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A.M. and P.M. and the 24-Hour Clock

A.M. and P.M. are abbreviations that tell us whether the time occurs in the morning or in the afternoon/evening.

  • A.M. occurs in the morning. It stands for ante meridiem which means “before midday”. You can think of it as “before noon”.

  • P.M. occurs in the afternoon and evening. It stands for post meridiem which means “after midday”. You can think of it as “after noon”.

In the A.M./P.M. system of time, a 12-hour clock is used. That means that the morning goes up until 12:00 noon and it starts over again with 01:00 and goes through 12:00 midnight.

In contrast, a 24-hour clock does not start over after 12:00 noon. The next hour (which we normally think of as 1:00 P.M.) is 13:00, then 14:00 etc. Time goes all the way up until 24:00 midnight which can also be called 00:00 because it is the start of the next day.

Let's look at a few examples:

 

1. What is 7:00 P.M. on a 24-hour clock?

    7:00 P.M. is after noon, so this is 7 hours past 12:00.

    So we will add: 12 + 7 = 19

    Answer: 19:00

    Note that A.M. and P.M. are not used with a 24-hour clock.

2. It is 21:10 hours. What time is it on a regular 12-hour clock?

    It is past noon, so let’s subtract 12 hours to see how far past noon it is.

    21 – 12 = 9, so it is 9:10 (The minutes won’t change),

    It is P.M. because it is after noon.

    Answer: 9:10 P.M.

3. On a 24-hour clock, it is 06:55. What time is it on a 12-hour clock with A.M./P.M.?

    Since it’s less than 12:00, the time will not change.

    This is before noon, so it is A.M.

    Answer: 6:55 A.M.

4. What is 12:05 P.M. on a 24-hour clock?

    12:05 P.M. is just barely in the afternoon, but we don’t need to change the time at all            

    because it is hasn’t yet started over with 1:00.

   Hence, the answer is: 12:05

    This makes sense because if we were to subtract 12 hours and get 00:05, this would

    only be 5 minutes into the day. That’s the time right after midnight not right after noon.

Summary

A.M./P.M. & 24-Hour Clock

  • A.M. occurs in the morning. It stands for ante meridiem which means “before midday”. You can think of it as “before noon”.

  • P.M. occurs in the afternoon and evening. It stands for post meridiem which means “after midday”. You can think of it as “after noon”.

  • A 24-hour clock does not start over after 12:00 noon. The next hour (which we normally think of as 1:00 P.M.) is 13:00, then 14:00 and so on and so forth.
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