How to teach 12 and 24 hour time

Grade 5 to Grade 6

The 24-hour clock counts the hours straight through from 00:00 to 23:59, so there is no am or pm and no repeating numbers. It is used on timetables, computers and in the military. Converting to and from 12-hour time comes down to the afternoon rule: after midday, add 12 to the hour.

How to teach it

1. Show why it exists: 3 o'clock is ambiguous, but 03:00 and 15:00 are not, which is why timetables use it.
2. Teach that morning hours match (9 am is 09:00) and always use two digits for the hour.
3. Teach the afternoon rule: from 1 pm onwards, add 12 to the hour (2 pm is 14:00, 8 pm is 20:00).
4. Handle the two tricky boundaries: midnight is 00:00 and midday is 12:00.
5. Practise both directions with real timetables, and work out durations in 24-hour time.

Worked example

Write 2:45 pm in 24-hour time:

   afternoon, so add 12 to the hour
   2 + 12 = 14
   2:45 pm = 14:45

Common mistakes

• Adding 12 to morning times as well as afternoon ones.
• Getting midnight and midday the wrong way round (midday is 12:00, midnight is 00:00).
• Writing a pm time without adding 12 (13:00 written as 1:00).
• Adding 12 to the minutes instead of the hour.