The 24-hour clock 1

1. The 24-hour clock 1 12-hour clock The hours go from 12 to 12 twice a day You must use am or pm: am is before noon pm is after noon 24-hour clock The hours go from 0 to 23 Time is always shown as four digits You don’t use am or pm Used in timetables, mobiles and computers

2. The 24-hour clock 2 After 12 o’clock in the afternoon, the hours continue to 13.00 = 1pm, 14.00 = 2pm, 15.00 = 3pm, up to 23.00 = 11pm. After 23, the hour goes to 00.00 = 12am then through 01.00 = 1am, 02.00 = 2am all the way to 23.00 = 11pm then 00.00 again. Because all the hours have a different number, there is no need to use am or pm. For example there are two 4 o’clocks in a day: one in the morning 4:00 am = 04:00 one in the afternoon 4:00 pm = 16:00 An easy way to change to 24-hour time is to add 12 onto the hour. You only do this to the hours between 1pm and 11pm. Midnight becomes 00:00.

3. The 24-hour clock 3 Fill in the 24-hour times on the scale below on your worksheet. 12-hour time am pm 12 1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 10 11 12 00.00 24-hour time

4. 12 hour 24 hour 3.00am 03:00 8.15am 08:15 11.45am 11:45 2.35pm 14:35 5.20pm 17:20 11.38pm 23:38 12.55am 00:55 The 24-hour clock 4 When converting to 24-hour time you change the hours only. Notice that you only add 12 onto the pm times.

5. The 24-hour clock 5 Change the following times from 12 hour into 24 hour. • 05:00 • 08:00 • 02:45 • 00:02 • 11:38 • 12:01 • 13:29 • 19:00 • 22:45 • 23:07 • 5am • 8am • 2.45am • 12.02am • 11.38am • 12.01pm • 1.29pm • 7pm • 10.45pm • 11.07pm Now check your answers.

6. The 24-hour clock 6 To convert from 24 hour to 12 hour you do the opposite. Take away 12 from the hours if it is 13 to 23. Midnight changes from 00:00 to 12:00am. Change the following times from 24 hour into 12 hour. • 4am • 7am • 9.45am • 12.20am • 11.02am • 12.56pm • 4.22pm • 8.20pm • 9.21pm • 10.18pm • 04:00 • 07:00 • 09:45 • 00:20 • 11:02 • 12:56 • 16:22 • 20:20 • 21:21 • 22:18 Now check your answers.

7. Time intervals 1 To calculate time intervals, you need to count forward. You might want to calculate time intervals for a variety of reasons. You might want to know how long a film lasts, how long a journey is going to last, how long you are in each class for in school. Can you think of any other reasons why you might want to calculate a time interval? To calculate the interval between 4.00am and 7.00am you count Forward. Start at 4 and count forward to 7 = 3 hours. + 1 + 1 1 4 5 6 7

8. 7. Time intervals 2 Find out how long there is between each of these times • 5.00pm and 8.00pm • 11.00pm and 4.00am • 3.30pm and 6.30pm • 22:40 and 00:40 • 07:05 and 12:05 • 12:55 and 14:55

9. 7. Time intervals 3 Find out how long there is between each of these times • 3.15pm and 5.15pm • 9.05pm and 2.05am • 7.15pm and 10.15pm • 14:10 and 23:10 • 17:25 and 22:25 • 13:05 and 18:05

10. Time intervals 4 To find out how many minutes there are between two time intervals and the times are in the same hour, take away the minutes. For example: 4.05 to 4.45 45 – 5 = 40 minutes The times are in the same hour, 4. This means that you can take away the minutes to calculate the time interval. 6.20 to 6.55 55 - 20 = 35 minutes 14:05 to 14:25 25 – 5 = 25 minutes

11. 7. Time intervals 5 Find out how long there is between each of these times: • 5.05pm and 5.20pm • 11.00pm and 11.45pm • 3.30pm and 3.55pm • 22:40 and 22:55 • 07:05 and 07:45 • 12:45 and 12:55

12. Time intervals 6 Calculating time intervals in minutes from one hour to the next can be broken down into two steps. Step 1 Count the minutes to the next hour Step 2 Add on the minutes to the finish time How long is it in minutes between: 3.50 pm and 4.25 pm 10 Step 1 10 minutes till 4 o’clock Step 2 4.00 – 4.25 = 25 minutes +25 35 minutes How long is it in minutes between 6.25 pm and 7.15 pm 35 Step 1 35 minutes till 7 o’clock Step 2 7.00 – 7.15 = 15 minutes +15 50 minutes

13. 7. Time intervals 7 Find out how long there is between each of these times: • 5.55pm and 6.20pm • 11.35pm and 12.20pm • 3.30pm and 4.05pm • 22:40 and 23:15 • 07:10 and 08:05 • 12:45 and 13:35

14. Time intervals 8 Remember that there are 60 minutes in an hour. If the time interval adds up to more than 60 minutes, change it into hours and minutes. How long is it in minutes between: 3.40 pm and 4.55 pm Step 1 20 minutes till 4 o’clock Step 2 4.00 – 4.55 = 55 minutes 20 +55 75 minutes 75 minutes = 1 hour (60 minutes) and 15 minutes How long is it in minutes between: 6.25 pm and 7.45 pm Step 1 35 minutes till 7 o’clock Step 2 7.00 – 7.45 = 45 minutes 35 + 45 80 minutes 80 minutes = 1 hour (60 minutes) and 20 minutes

15. 7. Time intervals 9 Find out how long there is between each of these times: • 5.50pm and 6.55pm • 11.35pm and 12.50pm • 3.30pm and 4.45pm • 22:40 and 23:55 • 07:10 and 08:45 • 12:45 and 13:55

16. + 2 hours + 25 minutes Time intervals 10 To calculate time intervals between hours follow these steps. 4.15 pm to 7.25 pm. 45 minutes 4.15 pm to 5.00 pm 5.00 pm to 7.00 pm 7.00 pm to 7.25 pm 45 + 25 70 minutes 70 minutes = 1 hour (60 minutes) and 10 minutes + 2 hours = 3 hours 10 minutes 19.20 pm to 21.55 pm. + 55 minutes 40 minutes + 1 hour 21.00 to 21.55 19.20 to 20.00 20.00 to 21.00 40 + 55 95 minutes 95 minutes = 1 hour (60 minutes) and 35 minutes + 1 hour = 2 hours 35 minutes

17. 7. Time intervals 11 Find out how long there is between each of these times: • 5.50pm and 8.55pm • 11.35pm and 6.50pm • 3.30pm and 12.45pm • 22:40 and 00:55 • 07:10 and 12:45 • 12:45 and 20:55

18. Time intervals 12 It is also very useful to be able to calculate time intervals in days, weeks and months. You may want to know how long it is until your birthday, or how long it is until your holidays or Christmas. Can you think of any other reasons why you might want to calculate time intervals using a calendar?

19. Calendars 1 You can calculate how many weeks and days there are between two dates by counting down the column. How many weeks is it between the 5th and the 26th? = 3 weeks How many weeks and days is it between the 5th and the 30th? = 3 weeks and 4 days

20. Calendars 2 1. How long is it between January 7th and 21st? 2. How long is it between January 1st and 29th? 3. How long is it between January 13th and 27th? 4. How long is it between January 19th and 26th? 5. How long is it between January 21st and 28th?

21. Calendars 3 1. How long is it between April 1st and 23rd? 2. How long is it between April 1st and 9th? 3. How long is it between April 4th and 27th? 4. How long is it between April 7th and 30th? 5. How long is it between April 10st and 22nd?

22. Time intervals between months 1 How long is it between January 15th and February 12th? 4 weeks (you don’t include the 15th) How long is it between January 15th and February 14th? 4 weeks and 2 days

23. Time intervals between months 2 How long is it between the following dates: 1. January 1st and February 23rd? 3. January 28th and February 13th? 4. January 31st and February 28th? 2. January 21st and February 28th?

24. Time intervals between months 3 How long is it between the following dates: 1. January 11th and February 4th? 3. January 18th and February 27th? 4. January 27th and February 24th? 2. January 26th and February 19th?

25. Time intervals between months 4 How long is it between the following dates: 1. January 4th and February 1st? 3. January 15th and February 23rd? 4. January 18th and February 1st? 2. January 13th and February 2nd?