# AM and PM and The 24 Hours Clock

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CCSS.Math.Content.4.MD.A.1

## AM and PM

AM and PM are abbreviations that tell us whether the time occurs in the morning or in the afternoon/evening.

• AM occurs in the morning. It stands for ante meridiem which means "before midday". You can think of it as "before noon".
• PM 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 AM/PM 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 PM) 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:

### Example 1

What is 7:00 PM on a 24-hour clock?

### Solution

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

So we will add: 12 + 7 = 19

### Example 2

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

### Solution

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 PM because it is after noon.

### Example 3

On a 24-hour clock, it is 06:55. What time is it on a 12-hour clock with AM/PM?

### Solution

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

This is before noon, so it is AM

### Example 4

What is 12:05 PM on a 24-hour clock?

### Solution

12:05 PM 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.

Summary

AM/PM & 24-Hour Clock

• AM occurs in the morning. It stands for ante meridiem which means "before midday". You can think of it as "before noon".
• PM 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 PM) is 13:00, then 14:00 and so on and so forth.

## More Date and Telling Time Lessons

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