Lesson video

Hi there, my name is Mr. Tilston.

I'm a teacher.

I teach all of the different subjects, but my favourite one just has to be maths.

So it's my great pleasure and great delight today to be teaching you a lesson all about time.

Time can be quite a tricky concept, there's a lot to know, there's a lot to remember, there's a lot to think about.

But don't worry, I'm here to guide you every step of the way, and remember, practise makes perfect.

So if you're ready, I'm ready.

Let's begin.

The outcome of today's lesson is I can convert between times given in 12 and 24 hours, and you might have had some very recent experience at looking at 12 and 24-hour digital times.

Today, we're going to be looking at converting from one to the other.

Our keywords today, my turn, 24-hour clock, your turn.

My turn, am, your turn.

And my turn, pm, your turn.

You might be familiar with those words, but let's have a look what they mean, let's have a reminder.

So 24-hour clock is a way of expressing the time using the numbers zero to 23 for the hours.

Am is the hours between midnight and midday, and pm is the hours between midday and midnight.

Our lesson today is split into two cycles, the first will be converting from 12-hour to 24-hour time and the second converting from 24-hour to 12-hour time.

So if you're ready, let's start by looking at converting from 12-hour to 24-hour time.

In this lesson, you're going to meet Sofia and Jun, have you met them before? They're here today to give us a helping hand.

The hour hand on an analogue clock is the first number on a digital clock.

So on your left, you've got an analogue clock, and on your right, you've got a digital clock.

So can you see the number two represented on both? And the minute hand on an analogue clock is the second number on the digital clock.

So you hopefully can see 25 minutes past on the analogue clock and you can see also 25 minutes past on the digital clock.

It tells you how many minutes have gone past the hour.

So that's how we would read that time in 12-hour digital time, 2:25 pm.

The same time can also be expressed on a 24-hour digital clock.

So we can say that is 14:25, we don't need to say pm.

In fact, we don't say pm.

Now, home time at Jun's school, which is the Oak Academy, is 10 minutes past three, is that similar to your home time? This can be expressed on an analogue clock, which you can see there, on a 12-hour digital clock using pm, or on a 24-hour digital clock.

So there we go, that's a 12-hour digital clock, 3:10 pm.

There's that 10 minutes passed and there's a three.

And we could also say that as 15:10.

That's in 24-hour digital clock time.

And again, we don't use the pm.

See if you can predict the next time when expressing times in both 12-hour and 24-hour form.

So our 12-hour digital clock and our 24-hour digital clock are both showing midnight, and that's how we write it.

And this is 1:00 am.

Now, did you notice how I said that versus how it's written? So I said it without any zeros, but it's written with zero, so it's written one colon 00 am, that's 1:00 am.

In 24-hour digital time, similar but not quite the same.

This is zero 100 hours, that's how we say it, but that's how it's written.

So a zero, a one, a colon, a zero, and a zero.

Now, did you notice there was no am there? There's no am or pm at all in 24-hour digital times.

And then we've got 2 am, zero 200 hours.

What do you notice about the am times when they're in 24-hour form? They're exactly the same, just without the am, so it's quite easy to convert those am times from 12-hour to 24-hour, not too much of a difference.

And that's the rest of those times, so have a look at those.

They're pretty similar, aren't they? You do need the zero in front of the 24-hour digital times for those times.

Let's keep going then, so this is 7:00 am, that's how we say it and that's how we write it in 12-hour digital form.

And in 24-hour form, it's zero 700 hours, that's how we say it and that's how we write it.

And here are some more times, now, you might notice that, by the time it gets to 10:00 am, we'd say that in 24-hour digital time as 10 hundred hours and 11:00 am, 11 hundred hours.

Now, the same is true of 12:00 pm, that's quite a special time, it's got its own set of rules.

So that's 12:00 pm, and there's no change, but that's the only time we can say that about the pm hours.

So 12:00 pm is 12 hundred hours.

Now, watch what happens with the other pm times because here, we do make a change from now on.

So this is 1:00 pm and that's 13 hundred hours.

2:00 pm, what do you think it's going to be? 14 hundred hours.

3:00 pm, 15 hundred hours.

4:00 pm, 16 hundred hours.

See if you can predict before it comes up.

5:00 pm, 17 hundred hours.

6:00 pm, 18 hundred hours.

What do you notice about the pm times when they're in 24-hour form? What change have we made? Two changes, fairly.

We add 12 to the hours on the pm time to turn it into a 24-hour time.

This is because it's 12 hours later in the day.

And the second change is we remove the pm.

Here we go, look, so here are the hours in the 12-hour form, and then we add 12 hours onto those to give us the hours in 24-hour form, so it's all about adding 12.

Let's do a check, add 12 hours to the pm time to turn it into a 24-hour time.

Pause the video and give that a go.

How did you find that? Did you find a little pattern there? Okay, so 7:00 pm, add 12 hours onto that seven, and it gives us 19 hundred hours.

8:00 pm, add 12 hours, that's 20 hundred hours.

9:00 pm, add 12 hours, that's 21 hundred hours.

10:00 pm, add 12 hours, is 22 hundred hours.

And then 11:00 pm, add 12 hours, is 23 hundred hours, and that's how we write that, well done if you got that.

The relationship between 12 and 24-hour times after midday can be explored using bar models.

So have a look at this bar model.

What's it showing? Well, let's have a look.

So the 13, that is the 24-hour time in hours, so 13 hundred hours.

This is showing midday, the 12 is representing midday, and the one is representing the hours after midday.

So that's showing midday plus one more hour, gives us 13 hundred hours.

So 13 equals 12 plus one.

Can you see that on the bar model? So 13 hundred is the same as 1:00 pm.

Here, we have 12 plus two, giving us 14.

So that's 14 equals 12 plus two, so 14 hundred is the same as 2:00 pm.

So that was midday plus two hours gives us 14 hundred hours.

What about this one? What can we see here? So 15 equals, is the same as 12 plus three.

So therefore, 15 hundred is the same as 3:00 pm.

So in this case, we've got midday plus three extra hours gives us 15 hundred hours.

Use the bar model to help convert 9:00 pm into 24-hour time, so you've got two of those parts of the bar model already given.

What is the missing part? Pause the video.

How did you get on? Really, it's asking what 12 plus nine is.

Hopefully, you got that.

So 21 is the same as 12 plus nine.

So therefore, 9:00 pm is the same as 21 hundred hours.

Let's have another check.

Sofia says 7:00 pm in 24-hour time is 17 hundred hours.

What feedback or advice can you give to Sofia? And if you've got somebody with you, chat it through with them, pause the video.

What can we say, maybe you had some good things to tell Sofia because she's not completely wrong there, she's got some little bits right.

Well, she has remembered that you don't use am or pm, so well done, Sofia, on that, when you tell me the 24-hour time, good stuff.

However, she's added 10 to the hours, not 12.

It should be 19 hundred hours, so very well done if you knew that, you're on track.

Adding 12 hours, adding 12 to the hours works when converting various pm times into 24-hour times, not just the o'clock times.

So up until this point, we've just looked at o'clock, well, here's some different times, but we still use the same rule.

So we focus on the hours, not the minutes when converting.

Here, look, we are looking at 1:07 pm.

Well, if we add 12 to one, we get 13.

So 1:07 pm is the same as 13:07.

6:23 pm.

Add 12 to the six and we get 18, so 6:23 pm is 18:23, and then 11:48 pm, we don't need to think about the 48, we do need to think about the 11, going to add 12 to it, 12 hours to that gives us 23.

So 23:48 is the same as 11:48 pm.

So really, it's all about adding 12.

Let's have a quick check, shall we? Convert these 12-hour times into 24-hour times.

Be careful, the first one is am and the other two are pm.

Pause the video and give that a go.

How did you get on? Let's have a look.

So the first one, 6:00 am, we don't need to make much of a change to that because it's an am time.

So we just need to add a zero to the start of it and take away that am.

You can see they're very similar, not much of a conversion, really.

But 7:10 pm, that tells us we do need to make a change, specifically to the seven.

We're going to add 12 hours onto that, we're not going to change the minutes, just the hours, so that becomes 19:10.

And then 10:37 pm, again, it's a pm time, so that tells us we do need to make a change to the hours, not the minutes, the hours.

So to the 10, we're going to add 12, and 10 add 12 is 22, so that's 22:37.

If you've got those three, well done, you are on track in today's lesson, you're doing really, really well.

Let's have another quick check.

So what feedback or advice can you give to Jun? And again, you might have some positive feedback for him as well as some suggestions.

So he says 2:10 pm in 24-hour time is 2:22 pm.

Pause the video.

What would you say to him? I can see there that Jun's got a bit of knowledge, but he's just used the wrong knowledge, really.

So well done, Jun, for remembering to add 12, he knew something about adding 12, didn't he, when converting from a 12-hour pm time to 24-hour time, but he didn't do it in the right place, he added 12 to the minutes instead of the hours.

Also, you don't need to use am or pm when using the 24-hour clock, in fact, you don't use them at all.

The correct time is 14:10, so well done if you said anything like that.

Let's do some practise.

Convert these 12-hour times into 24-hour times.

And then just a little something to think about from Sofia, she says, "Do you need to add 12 hours to am and pm times?" Got some more here for B.

And number two, Jun and his family need to catch a train to Nottingham.

It leaves at 18:05, but that's the time they arrive at the station, so look at that clock.

Will they be able to catch the train or are they too late? Explain.

Pause the video, and I'll see you soon for some feedback.

Welcome back, how did you get on? Well, number one, so 4:00 am is 04 hundred hours, that's how we write that, 8:55 am is 8:55, that's how we write that, remember the zero at the front.

2:00 pm is 14 hundred hours, so we've made a conversion there.

9:30 pm is 21:30, and 10:12 pm is 22:12.

10:00 pm is 22 hundred hours, 7:18 am is 7:18 with a zero on and no am.

2:00 pm is 14 hundred hours, 12:30 pm is 12:30, so remember that 12s are special, that's the only pm time that you don't convert.

3:28 pm is 15:28, and 8:59 pm is 20:59, well done if you got those.

And Jun and his family need to catch a train to Nottingham, it's a lovely place, by the way.

It leaves at 18:05, this is the time they arrive at the station.

Will they be able to catch the train or are they too late? Well, they will be able to catch the train, phew.

The time is five minutes to six, that's how we'd say that on an analogue clock, and the train leaves at five minutes past six, so they've got 10 minutes, I think they've got time to grab a quick sandwich, what do you reckon? Okay, you're doing really well, it's time to move on to the second cycle, which is converting from 24-hour to 12-hour time, let's go.

So to go from 1:07 pm to 24-hour time, we'd add 12 hours, so we've done lots of practise doing that.

Subtraction is the inverse, the opposite of addition.

So to convert from 24-hour time back to 12-hour time, we use the inverse.

So what's the opposite of add? Subtract.

So instead of adding 12 hours, what are we going to do? We're going to subtract 12 hours.

So the 13, we're going to take 12 off, and that gives us one, so subtract 12, that gives us one, so now we've gone from the 24-hour time to the 12-hour time.

What will the 12-hour digital times be for these 24-hour times? So let's start with 23:00 hours, so we're going to subtract 12 from that 23.

23 take away 12 is 11, so that's 11:00 pm.

Again, let's go from a 24-hour time to a 12, let's do 22 hundred hours.

22 take away 12 is 10, so that's 10:00 pm.

21 hundred hours, take away 12, 9:00 pm.

20 hundred hours take away 12 is 8:00 pm.

And then 19 hundred hours, take away 12, what's that going to be? 7:00 pm.

So this time, we've gone the other way around, we've gone from 24-hour time to 12-hour time.

Let's have a quick check.

Match the 12 and 24-hour digital time, which give the same time.

Remember, you're going to be subtracting 12 hours from those 24-hour times to give you the 12-hour times.

Pause the video and give that a go.

Let's have a look.

So 15 hundred hours, 15 take away 12 is three.

So those two are pairs, so they're the same time.

18 take away 12 is six, so those two are the same time.

13 take away 12 is one, so 13 hundred hours and 1:00 pm are the same, 16 takeaway 12 is four, so they are the same time.

17 take away 12 is five, so 5:00 pm and 17 hundred are the same time.

And last but not least, 14 take away 12 is two.

So 14 hundred hours is 2:00 pm.

Just like before, there is no need to convert the am times or that special 12:00 pm time, they remain the same, no conversion needed, we're just taking away the words am and in the case of 12, the word pm.

Remember to add the zero to the front of those first times as well.

Another strategy is to count on from 12 o'clock to the 24-hour time, let's have a look at that.

So we'll bring back those bar models, okay.

What's that showing, look at that bar model.

That's showing that 12 plus something equals 13, or 13 take away 12 is something.

So 13 equals 12 plus one.

So 13 hundred is the same as 1:00 pm.

Let's have a look at this one.

What's this showing? 14 equals what? 14 equals 12 plus two.

So 14 hundred is the same as 2:00 pm.

Let's do another one.

What about this one? What's this showing? This is showing that 15 equals 12 plus five, so therefore, 17 hundred hours is the same as 5:00 pm.

So in all of these cases, we're adding a certain number of hours onto midday.

Let's have a check, let's see if you've got that.

Use the bar model to help convert 20 hundred hours into 12-hour time.

Pause the video.

So we're essentially saying 20 equals 12 plus what? And that will give us the 12-hour time.

20 equals 12 plus eight.

So 20 hundred hours is the same as 8:00 pm.

It's time for some practise, some final practise.

Number one, fill in the blanks.

Number two, convert these 24-hour times into 12-hour times, and Sofia's given you a little hint or reminder there, how do you convert 24-hour times to pm times? Got some more to do there.

And a little bit of a problem.

Sofia and her family are watching a film at the cinema.

It starts at 17:50.

This is the time they arrive at the cinema.

Have they missed the start of the film? Explain.

Pause the video, good luck with that, and I'll see you soon for some answers.

Welcome back, how did you find that? How did you get on? Let's have a look.

So filling in the blank, so the missing number in the bar model is nine, so 21 equals 12 plus nine.

So 21 hundred hours is the same as 9:00 pm.

And then converting those 24-hour times into 12-hour times, 7:00 am and seven or 07 hundred hours, 12:53 pm, that's 12:53, 10:00 pm is 22 hundred hours, 8:10 pm is 20:10, and 4:12 pm is 16:12.

So in all of those cases, we've taken the 24-hour time, the hours part of it, and subtracted 12.

And Sofia's tip, you only need to count on from 12 or subtract 12 hours from the pm times.

So for B, 11:00 am, 11 hundred hours, 11:03 pm, 23:03, 3:15 pm, 15:15, 1:38 pm, 13:38, 6:29 pm, 18:29, 12:46 pm, 12:46.

No change to that one, that's a special one.

And Sofia and her family are watching a film at the cinema, and it starts at 17:50, that's the time they arrive, 7:45 pm, have they missed the start of the film? Yeah, they're very late, in fact, the film might even be over.

We've come to the end of the lesson.

Our lesson today has been converting between times given in 12 and 24 hours.

For all times from 1:00 pm onwards, add 12 hours to get the 24-hour time and subtract 12 hours to go from the 24-hour time to the 12-hour time.

So we can see that, in this image here, 2:10 pm plus 12 hours is 14:10, and then 14:10 take away 12 hours is 2:10 pm.

So it's all about adding and subtracting 12.

You've been amazing today, you've made lots and lots of progress, and it's been a real pleasure working with you.

I hope I get the chance to work with you again in the future.

But until then, well done on today's achievements, give yourself a pat on the back.

Enjoy the rest of your day, whatever you've got in store, take care and goodbye.