In 12-hour time the day is split in two: am is from midnight to midday, and pm is from midday to midnight. In 24-hour time the hours keep counting after midday: 1 pm is 13:00, 2 pm is 14:00. So for a pm time, add 12 to the hour to get 24-hour time, and take 12 away to go back. Two hours are special. The hour after midnight is 12 am, written 00 in 24-hour time. The hour after midday is 12 pm, which stays as 12. 24-hour times never use am or pm.
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Move the hands
3:00
The short hand moves between hour numbers as the minutes pass. One full turn of the long hand is sixty minutes.
Words to know
- am
- the half of the day from midnight to midday
- pm
- the half of the day from midday to midnight
- 24-hour time
- a way of writing time where the hours go from 00 to 23
Worked examples
Write 15:40 in 12-hour time.
- 1.15 is after 12, so it is pm.
- 2.15 − 12 = 3.
= 3:40 pm
Write 12:25 am in 24-hour time.
- 1.12:25 am is just after midnight.
- 2.The day starts at 00:00, so the hour 12 am is written 00.
= 00:25
A movie starts at 7:40 pm and lasts 1 h 35 min. Write the finish time in 24-hour time.
- 1.7:40 pm + 1 h = 8:40 pm. 40 + 35 = 75 minutes, which is 1 hour 15 minutes, so 9:15 pm.
- 2.9 + 12 = 21.
= 21:15
For parents
Method: Add or take away 12 for pm hours
What to say: Is this time in the morning or the afternoon? Then should we add 12 or not?
Common mistakes:
- Takes away (or adds) 10 instead of 12 when changing between 24-hour and am/pm time.
- Mixes up am and pm (including midnight and midday), so the time lands in the wrong half of the day.
- Mixes the two systems: writes the 24-hour hour and adds am or pm as well.
- Applies the 12-hour rule to the hours around midnight and midday too (12:25 pm → 24:25, 12:25 → 0:25 pm).
- Copies the 00 hour into 12-hour time as 0 instead of calling it 12 am.
- Adds minutes as if an hour had 100 minutes, so the answer has 60 or more minutes.