Lesson 18 — Activity 1: From Standard to 24-Hour Time

Getting Ready

Have you ever heard anyone say that they will meet you at 1800 hrs? Did you understand what they meant or did you think they were being silly?

While they might have been having a bit of fun with you, they were actually telling you when they would meet you. All they have did was use the 24-hour clock to tell you what time they would arrive!

In this lesson, we will look at the 24-hour clock and in this activity you will learn how to convert between the 24-hour clock and the 12-hour clock.     

Courtesy of Pixabay


Think about This:

What kind of watch or clock do you use most often? If you use one that looks like this,

you are using an analog timepiece. If you are using one that looks like this,

you are using a digital clock or watch.   

Courtesy of Pixabay

Click here to review telling time using an analog and digital clock. Complete Topics 1 – 3.

From Standard to 24-Hour Time 

A standard clock is divided into 12 hours. If you start at midnight, the first time around the clock is called a.m., which means ante meridiem. This is Latin for "before noon." And the second time around the clock we call p.m., which means post meridiem, which is Latin for "pre-midnight." Sometimes this can get confusing.

Courtesy of Pixabay

If you are talking to someone you don't know very well and they say they will meet you at 8, do they mean 8 a.m. or 8 p.m.?

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What about filling out a form and forgetting to write down the a.m. or p.m.? Did the patient get the medicine at 8 a.m. or 8 p.m.? It is very important to know so that the next person working with the patient doesn't either miss a dose, or worse yet, doesn't give the patient too much medicine!

One solution to this problem is the 24-hour clock. After all, there are 24 hours in a day, so why not divide up the day into 24 hours? People already do this, but in two sets of 12-hour groups.

So how does this work?
 Start at midnight. Midnight is 0000 hours, 1 a.m. is 0100 hours — say this as zero one hundred hours; noon is 1200 hours and after that, we just start to add on until we get to midnight again. Midnight can also be called 2400 hours, 1 p.m. is 1300 hours; 2 p.m. is 1400 hours and so on.

Does this sound a bit confusing to you? It shouldn't. With a little practise, it is very easy and eliminates ANY confusion! With a 24-hour clock, there is no way that you can get mixed up between 8 a.m. and 8 p.m.
 8 a.m. is 0800 hours and 8 p.m. is 2000 hours.

One of the biggest advantages to using the 24-hour clock is that it helps avoid mistakenly setting digital or analog alarm clocks for the p.m. instead of the a.m. hour or vice versa.

Another benefit of the 24-hour clock is in helping you figure out how long until something occurs.

Courtesy of Pixabay

Let's say you have a really big event this evening at 9 p.m., or 2100 hours.
 You have a lot of things to do beforehand. Right now it is 10 a.m., or 1000 hours. How many hours until the event starts?

 If you go with standard time, we have to add 2 hours to get to noon and then add another 9 hours to get to 9 p.m. That is a total of 11 hours. Because you are doing two steps to get the answer, there is room for error, and you don't want to be showing up an hour late for your event.

But what if you figure it out using the 24-hour clock? You only have to do one step, 2100 minus 1000 is 1100. In one step we know that there are 11 hours until the event!

To convert from a standard clock to a 24-hour clock is very easy.

 Changing anything from midnight to noon is as simple as writing the time in a four-digit number without any colon between hours and minutes. You simply fill in any blanks with zeros.

For example, 8:10 a.m. becomes 0810 hours. 11:15 a.m. becomes 1115 hours.

Once you are past noon, you simply add whatever hour it is on the standard clock to 12. This makes 3:15 p.m. 1515 hours: 12 plus 3 is 15.

Please note that this time may be written as 15:15. As well, actually saying the words is an option too.  

Try This:

Click here to practise using the 24-hour clock. Complete Topics Four and Five in the video.


Try This!

Click here to try a Self-check Activity to practise converting to the 24-hour clock.


Digging Deeper

Click here to try a game to read both analog and digital times. Begin at Topic Six.