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
- Show why it exists: 3 o'clock is ambiguous, but 03:00 and 15:00 are not, which is why timetables use it.
- Teach that morning hours match (9 am is 09:00) and always use two digits for the hour.
- Teach the afternoon rule: from 1 pm onwards, add 12 to the hour (2 pm is 14:00, 8 pm is 20:00).
- Handle the two tricky boundaries: midnight is 00:00 and midday is 12:00.
- 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.
Frequently asked questions
What is 24-hour time?
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, because a time like 15:00 is unambiguous where 3 o'clock is not.
What age or grade is 24-hour time taught?
The 24-hour clock is usually taught in Grade 5 and Grade 6, once telling the time is secure. Students learn to convert between 12 and 24 hour time, handle the midnight and midday boundaries, and work out durations using timetables.
How do you convert afternoon time to 24-hour time?
From 1 pm onwards, add 12 to the hour. So 2 pm becomes 14:00 and 8 pm becomes 20:00. Morning times stay the same, just written with two digits, so 9 am is 09:00. The afternoon rule is the key step in converting to 24-hour time.
What is midnight and midday in 24-hour time?
Midday is 12:00 and midnight is 00:00. These two boundaries are the trickiest part of 24-hour time and are often swapped. Getting them the right way round, midday at 12 and midnight at 00, is worth practising carefully.
Why does the 24-hour clock exist?
It removes ambiguity. In 12-hour time, 3 o'clock could be morning or afternoon, but 03:00 and 15:00 are clearly different. That is why timetables, computers and the military use it, since a single unambiguous number for each hour prevents mix-ups.
Why does my child struggle with 24-hour time?
Common errors are adding 12 to morning times as well as afternoon ones, getting midnight and midday the wrong way round, writing a pm time without adding 12, and adding 12 to the minutes instead of the hour. Practising the afternoon rule and the two boundaries fixes most of these.
How do you write 2:45 pm in 24-hour time?
It is 14:45. Because it is afternoon, add 12 to the hour: 2 plus 12 is 14, and the minutes stay the same. So 2:45 pm becomes 14:45. Remember to add 12 to the hour only, never to the minutes.
Practise with free worksheets
Printable worksheets with answer keys that are never wrong.