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24-hour time
24-hour time
We are used to telling time on clocks that are numbered 1 - 12.
Sometimes the clock represents the AM (which stands for Ante Meridiem),
But sometimes it represents the PM (which stands for Post Meridiem).
This is called 12-hour time.
...zzzt....You guys gotta move outta there. We're going to bomb the place at 6:00...zzzt...zzzt....got it?
uhh....could you repeat that?
In 1920, the US Navy adopted a standard way of telling time that didn’t use AM and PM. This system has since been adopted by many professions including nursing where giving medication at the proper time can be a life or death situation.
This system is called Military Time (or 24-hour time).
In Military time, you can think of the clock looking like this.
The ancient Egyptians were the ones who decided a day is 24 hours. (10 hours of day, 12 hours of night, and 2 hours of twilight between them)
If the folks who invented clocks made them like this, you would have learned 24-hour time in grade school.
However, confusion can arise if you aren't sure if a time is AM or PM.
But….
The people who invented clocks were bigger fans of the Babylonians than they were of the Egyptians.
The Babylonian Astronomers were the ones who figured out that a year is about 360 days.
They loved this, because their number system was based on 60 (and 360 is 60 x 6).
(The fact that there are 360 degrees in a circle came from this as well.)
They wanted to break up an hour into 60 bits called minutes (pars minuta prima) and
the minutes into 60 bits called seconds (pars minuta secunda) to continue the Babylonian thing.
…and this only works nicely on a 12 hour clock.
So we were stuck with 12 hour clocks.
How to convert 12-hour time to 24-hour time
How to convert 12-hour time to 24-hour time
Rules for writing it.
Rules for writing it.
Ok… So how do you convert a time on a 12-hour clock to military time?
For example, look at the time on this clock. It’s 2:30 AM.
For AM times, these are the rules:
1. Drop the colon (some people still use it, but officially you don’t. Also drop the "AM".
2. Military time has 4 digits. If the hour has only a single digit, add a 0 in front. (When reading the time you say “Oh” to show the 0 is there)
3. Midnight is 0000, so a time like 12:48 AM is 0048 (NOT 1248)
For example, look at the time on this clock. It’s 2:30 AM.
For example, look at the time on this clock. It’s 2:30 AM.
What is this time in military time?
2:30 AM ⇾ 230 ⇾ 0230
If this is in the AM, what is this time in 12-hour and 24-hour time?
If this is in the AM, what is this time in 12-hour and 24-hour time?
12-hour time: 6:42 AM
24-hour time: 0642
If this is in the AM, what is this time in 12-hour and 24-hour time?
If this is in the AM, what is this time in 12-hour and 24-hour time?
12-hour time: 12:19 AM
24-hour time: 0019
Rules for PM.
1. Drop the colon, drop the PM, use 4 digits.
2. Noon is 1200. For hours after that, add 12 to get the new number of hours.
2:00 PM is ____________
7:15 PM is ___________
10:56 PM is _____________
2:00 PM = 1400, 7:15 PM = 1915, 10:56 PM = 2256
If this is in the PM, what is this time in 12-hour and 24-hour time?
If this is in the PM, what is this time in 12-hour and 24-hour time?
12-hour time: 6:30 PM
24-hour time: 1830
If this is in the PM, what is this time in 12-hour and 24-hour time?
If this is in the PM, what is this time in 12-hour and 24-hour time?
12-hour time: 11:03 PM
24-hour time: 2303
Converting 24-hour time back to wimpy 12-hour time.
If the hour is 11 or less, then it is AM.
Add a colon and lose any zeros in the front.
If the hour is 12 or more, then it is PM.
If the hour is 13 or more, subtract 12 from it to get the new hour.
Add a colon.
Examples
Examples
1556 is __________________ AM or PM?
0732 is __________________ AM or PM?
2105 is __________________ AM or PM?
1556 = 3:56 PM, 0732 = 7:32 AM, 2105 = 9:05 PM
A couple more things…..
You may wonder, is 12 midnight 0000 or 2400?
Actually, it can be both.
If you are talking about a time at the end of the current day, use 2400.
If you are talking about a brand new day, use 0000.
Also, consider a nurse who has to give medication every 5 hours, starting at 4 AM.
In military time, it is easy to calculate when to give the medication.
24-hour time: 0400 ________ ________ ________ ________
12-hour time: 4:00AM ________ ________ ________ ________
24-hour: 0400, 0900, 1400, 1900, 2400
12 hour: 4:00 AM, 9:00 AM, 2:00 PM, 7:00 PM, 12:00 AM
Now it's time to do this check our understanding.
Now it's time to do this check our understanding.
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