The idea, step by step
Change the day when the clock passes midnight
A simultaneous event can occur on different local days. If adding the offset difference takes the clock to 24:00 or beyond, subtract 24 hours and move to the next day. If subtraction goes before 00:00, add 24 hours and move to the previous day.
Write the starting day before calculating. For Friday 23:20 at UTC−3, converting to UTC+9 adds 12 hours. The answer is Saturday 11:20, not Friday 11:20. The numerical clock reading alone leaves an important part of the answer missing.
Check the date as well as the hour
Some offset differences reach 24 hours. UTC+14 is a full day ahead of UTC−10, so the same event has the same clock reading but a different local date. For ordinary questions, use the stated fixed offsets and avoid assuming that every place shares the same calendar day.
To check, convert your answer back using both the time and day. In travel questions, local departure and arrival readings belong to different clocks; the apparent change in their dates is not automatically the flight duration. First put both readings onto one reference clock.
A common trap
If two clocks show the same hours and minutes, their dates must match.
Instead: Offsets can differ by 24 hours. The clock readings then match while the local dates differ by one day.
See the thinking
Move into the next day
A town at UTC−3 reads Friday 23:20. What is the simultaneous day and time at UTC+9?
- The difference is 9−(−3)=12 h.
- 23:20+12 h=35:20 on a continuous count.
- Subtract 24 h and advance Friday to Saturday.
Move into the previous day
A town at UTC+11 reads Monday 01:10. Find the simultaneous day and time at UTC−7.
- The destination is 18 h behind: −7−11=−18 h.
- 01:10−18 h crosses into the previous day.
- Add 24 h to obtain Sunday 07:10.
Check your understanding
UTC+2 reads Tuesday 23:30. Give the simultaneous day/time at UTC+5.
Show answer
Wednesday 02:30.
Add 3 hours: 30 minutes reaches midnight, then another 2 hours 30 minutes reaches Wednesday 02:30.
Your turn
6 questions · 12 marks · Work at your own pace.
Read the questions online
- UTC−2 reads Thursday 22:45. Find the simultaneous day/time at UTC+4.2 marks
- UTC+10 reads Tuesday 03:20. Find the simultaneous day/time at UTC+1.2 marks
- UTC+14 reads Wednesday 10:00. Find the simultaneous day/time at UTC−10.2 marks
- UTC−6 reads Sunday 20:15. Find the simultaneous day/time at UTC+5:30.2 marks
- UTC+3 reads Saturday 00:05. Find the simultaneous day/time at UTC−1.2 marks
- UTC+8 reads Monday 22:50. A simultaneous clock at UTC+11 is claimed to show Monday 01:50. Correct the full answer.2 marks
Read the worked solutions
Equivalent correct methods earn the corresponding marks. Include units where asked.
- UTC−2 reads Thursday 22:45. Find the simultaneous day/time at UTC+4.2 marks
Add 6 h; cross midnight once.
Friday 04:45. - UTC+10 reads Tuesday 03:20. Find the simultaneous day/time at UTC+1.2 marks
Subtract 9 h; cross back to Monday.
Monday 18:20. - UTC+14 reads Wednesday 10:00. Find the simultaneous day/time at UTC−10.2 marks
Subtract 24 h exactly.
Tuesday 10:00. - UTC−6 reads Sunday 20:15. Find the simultaneous day/time at UTC+5:30.2 marks
Add 11 h 30 min.
20:15+11:30=31:45; subtract 24 h.
Monday 07:45. - UTC+3 reads Saturday 00:05. Find the simultaneous day/time at UTC−1.2 marks
Subtract 4 h and go to the previous day.
Friday 20:05. - UTC+8 reads Monday 22:50. A simultaneous clock at UTC+11 is claimed to show Monday 01:50. Correct the full answer.2 marks
The 3-hour addition crosses midnight; the day must advance.
Tuesday 01:50.
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 ↗