The idea, step by step
Split at midnight
When a finishing time is on the next day, subtracting its clock reading from the start gives the wrong sign. Instead, count from the start to midnight, then from midnight to the finish. Add the two durations.
For 22:48 to 01:17 the next day, the first part is 1 hour 12 minutes, and the second is 1 hour 17 minutes. The full duration is 2 hours 29 minutes. State that the finish is next day; the clock alone repeats every 24 hours.
Add minutes carefully
To add a duration, first add hours and minutes, then regroup every 60 minutes as one hour. If the hour reaches 24 or more, subtract 24 and advance the day. For durations longer than one day, count complete 24-hour blocks as well.
Use a second method to check: convert both parts to minutes, or add your final duration back to the starting time. Decimal hours use tenths of an hour, not minutes; for example, 1.5 hours is 1 hour 30 minutes, not 1 hour 5 minutes.
A common trap
00:40−23:50=−23:10, so the journey has a negative duration.
Instead: The finish is next day. There are 10 minutes to midnight and 40 more afterwards: 50 minutes.
See the thinking
A late journey
Find the time from 22:48 until 01:17 the next day.
- 22:48 to 00:00 is 1 h 12 min.
- 00:00 to 01:17 is 1 h 17 min.
- Add: 1 h 12 min+1 h 17 min=2 h 29 min.
Finish after midnight
A job begins at 23:25 and lasts 2 h 50 min. When does it finish?
- 23:25+2 h=01:25 next day.
- Add another 50 min: 02:15.
- Check that the total time added is 2 h 50 min.
Check your understanding
How long is 23:40 to 00:25 the next day?
Show answer
45 minutes.
20 minutes to midnight plus 25 minutes after midnight gives 45 minutes.
Your turn
6 questions · 12 marks · Work at your own pace.
Read the questions online
- Find the time from 23:15 to 00:05 the next day.2 marks
- Find the time from 21:40 to 02:10 the next day.2 marks
- Add 1 h 35 min to 23:10.2 marks
- A shift runs from 18:30 to 01:00 the next day. Find its duration.2 marks
- An event finishes at 00:20 Tuesday after lasting 2 h 45 min. When did it start?2 marks
- Convert 2.5 hours to hours and minutes, then add it to 22:50.2 marks
Read the worked solutions
Equivalent correct methods earn the corresponding marks. Include units where asked.
- Find the time from 23:15 to 00:05 the next day.2 marks
45 min to midnight+5 min=50 min.
50 minutes. - Find the time from 21:40 to 02:10 the next day.2 marks
2 h 20 min+2 h 10 min=4 h 30 min.
4 h 30 min. - Add 1 h 35 min to 23:10.2 marks
50 min reach midnight; another 45 min remain.
00:45 next day. - A shift runs from 18:30 to 01:00 the next day. Find its duration.2 marks
5 h 30 min until midnight+1 h=6 h 30 min.
6 h 30 min. - An event finishes at 00:20 Tuesday after lasting 2 h 45 min. When did it start?2 marks
Subtract 20 min to 00:00 Tuesday; another 2 h 25 min takes you back to 21:35 Monday.
21:35 Monday. - Convert 2.5 hours to hours and minutes, then add it to 22:50.2 marks
0.5×60=30 min.
22:50+2 h 30 min=01:20 next day.
2 h 30 min; finish 01:20 next day.
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 ↗