The idea, step by step
Read the hour before changing it
In 24-hour time, the day begins at 00:00 and ends just before the next 00:00. Morning hours 01 to 11 match 1 am to 11 am. After noon, add 12 to the hour for pm times from 1 pm to 11 pm. The minutes do not change.
The exceptions are 12 am and 12 pm. Midnight is 00:00, so 12:25 am is 00:25. Noon is 12:00, so 12:25 pm stays 12:25. A leading zero, as in 07:05, helps keep the notation clear.
| 12-hour time | 24-hour time |
|---|---|
| Midnight | 00:00 |
| 7:05 am | 07:05 |
| Noon | 12:00 |
| 6:40 pm | 18:40 |
Convert back and check the part of day
For 24-hour hours 13 to 23, subtract 12 and use pm. Hours 01 to 11 use am. Hour 00 means 12 am; hour 12 means 12 pm. Do not subtract 12 from the minutes.
Finish with a sense check: 21:10 is late evening, so 9:10 pm is reasonable. If your conversion says am, revisit the hour. Include am/pm when giving a 12-hour answer; “9:10” alone does not tell us which half of the day you mean.
A common trap
12:30 am is written as 12:30 in 24-hour time.
Instead: 12:30 am is half an hour after midnight, so it is 00:30. 12:30 in 24-hour time is after noon.
See the thinking
Evening to 24-hour time
Write 6:35 pm in 24-hour time.
- It is after noon and the hour is not 12.
- Add 12 to 6 to get 18.
- Keep the minutes as 35.
Just after midnight
Write 00:12 in 12-hour time.
- Hour 00 is the hour beginning at midnight.
- Use 12 am, keeping the minutes 12.
Check your understanding
Write 12:45 pm in 24-hour time.
Show answer
12:45
The noon hour remains 12; adding 12 would incorrectly move it to the following midnight.
Your turn
6 questions · 7 marks · Work at your own pace.
Read the questions online
- Write 8:07 am in 24-hour time.1 mark
- Write 9:25 pm in 24-hour time.1 mark
- Write 12:10 am in 24-hour time.1 mark
- Write 17:42 in 12-hour time.1 mark
- Write 12:05 in 12-hour time.1 mark
- A student writes 7:50 pm as 07:50. Correct the time and explain.2 marks
Read the worked solutions
Equivalent correct methods earn the corresponding marks. Include units where asked.
- Write 8:07 am in 24-hour time.1 mark
Morning hour 8 remains 08; minutes stay 07.
08:07 - Write 9:25 pm in 24-hour time.1 mark
9+12=21; retain 25 minutes.
21:25 - Write 12:10 am in 24-hour time.1 mark
The midnight hour is 00.
00:10 - Write 17:42 in 12-hour time.1 mark
17−12=5; this is after noon.
5:42 pm - Write 12:05 in 12-hour time.1 mark
The 12 hour is noon, not midnight.
12:05 pm - A student writes 7:50 pm as 07:50. Correct the time and explain.2 marks
7+12=19 for the evening hour.
19:50. 07:50 is in the morning.
Curriculum reference & lesson scope
Australian Curriculum v9 and SA Curriculum Mathematics
Converting 12-/24-hour time; elapsed time across midnight; timetables and journeys; time-zone differences; date changes; checking real-world scheduling answers.
Official curriculum source ↗