Measures
Understanding Time: 24-hour Clock, Intervals, and Timetables
3rd Class · 4th Class · 5th Class · 6th Class
- ✓By the end of this lesson students will be able to convert between the 12-hour and 24-hour clock formats.
- ✓By the end of this lesson students will be able to calculate the duration of time intervals.
- ✓By the end of this lesson students will be able to read and interpret information from simple timetables.
- ✓By the end of this lesson students will be able to solve practical problems involving time.
Key concepts
The 12-hour clock uses numbers 1 to 12 to show the time, along with 'AM' (ante meridiem) for times from midnight to just before noon, and 'PM' (post meridiem) for times from noon to just before midnight. For example, 8:00 AM is eight o'clock in the morning, and 8:00 PM is eight o'clock in the evening.
The 24-hour clock uses numbers 00 to 23 to show the time in a single day, without using 'AM' or 'PM'. Times are written with four digits, e.g., 09:30 for half past nine in the morning, or 14:00 for two o'clock in the afternoon. Midnight at the start of a day is 00:00, and midnight at the end of a day can be 24:00.
To convert 12-hour times to 24-hour times: - For AM times (1:00 AM to 11:59 AM): The hour stays the same. (e.g., 9:30 AM is 09:30). - For 12:00 AM (midnight): This is 00:00 in 24-hour time. - For PM times (1:00 PM to 11:59 PM): Add 12 to the hour. (e.g., 3:45 PM is 15:45). - For 12:00 PM (noon): This is 12:00 in 24-hour time.
To convert 24-hour times to 12-hour times: - For times from 00:00 to 00:59: Subtract 12 hours and add 'AM'. (e.g., 00:30 is 12:30 AM). - For times from 01:00 to 11:59: The hour stays the same, add 'AM'. (e.g., 07:15 is 7:15 AM). - For times from 12:00 to 12:59: The hour stays the same, add 'PM'. (e.g., 12:40 is 12:40 PM). - For times from 13:00 to 23:59: Subtract 12 hours and add 'PM'. (e.g., 18:00 is 6:00 PM).
A time interval is the amount of time that passes between a start time and an end time. It is also called duration. To calculate an interval, you can count forward from the start time, or subtract the start time from the end time (being careful when minutes need to be borrowed from hours).
A timetable is a chart that shows the times when events happen, such as bus departures, train arrivals, or school lessons. Timetables help us plan our day and know when things are scheduled to happen.
Key facts to remember
- 1There are 24 hours in a day.
- 2There are 60 minutes in an hour.
- 3The 24-hour clock does not use AM or PM.
- 4Midnight is 00:00 (at the start of a day) or 24:00 (at the end of a day).
- 5To convert PM times (except 12 PM) to 24-hour clock, you add 12 to the hour.
- 6To convert 24-hour times (from 13:00 to 23:59) to 12-hour clock, you subtract 12 from the hour and add PM.
- 7A time interval is the duration or amount of time between a start time and an end time.
- 8Timetables are charts that show scheduled times for events.
Worked examples
Example 1
Convert the following times to 24-hour clock: a) 8:25 AM b) 4:10 PM
Answer
a) 08:25 b) 16:10
Remember to always use four digits for 24-hour time, e.g., 08:25, not 8:25.
Example 2
A bus departs at 09:40 and arrives at 11:15. How long was the journey?
Answer
The journey was 1 hour and 35 minutes long.
You can also subtract: 11 hours 15 minutes - 9 hours 40 minutes. You would borrow 1 hour (60 minutes) from 11 hours, making it 10 hours and 75 minutes. Then, 10 hours 75 minutes - 9 hours 40 minutes = 1 hour 35 minutes.
Example 3
Use the bus timetable below to answer the questions. | Bus Stop | Bus A | Bus B | Bus C | |---------------|-------|-------|-------| | Town Centre | 07:30 | 08:15 | 09:00 | | Park Gate | 07:45 | 08:30 | 09:15 | | Shopping Mall | 08:05 | 08:50 | 09:35 | 1. What time does Bus B arrive at the Shopping Mall? 2. How long does Bus C take to travel from Town Centre to Shopping Mall?
Answer
1. Bus B arrives at the Shopping Mall at 08:50. 2. Bus C takes 35 minutes to travel from Town Centre to Shopping Mall.
Always read the rows and columns carefully on a timetable to avoid making mistakes.
Common mistakes
- ✗Forgetting to add 12 to the hour for PM times when converting to 24-hour clock (e.g., writing 3:00 PM as 03:00 instead of 15:00).
- ✗Incorrectly converting 12 AM (midnight) to 12:00 in 24-hour time (it should be 00:00).
- ✗Incorrectly converting 12 PM (noon) to 00:00 in 24-hour time (it should be 12:00).
- ✗Making errors when calculating time intervals that cross an hour boundary (e.g., 10:45 to 11:15).
- ✗Misreading rows or columns when interpreting information from a timetable.
Exam tips
- ★Always check whether the question asks for your answer in 12-hour or 24-hour time.
- ★When calculating time intervals, it can be helpful to count up to the next full hour first, then count the full hours, and finally count the remaining minutes.
- ★For timetable questions, use your finger to trace the correct row and column to ensure you are reading the correct information.
- ★Practice converting times between 12-hour and 24-hour formats regularly to become quicker and more accurate.
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