Primary 3 Time
Keywords
- Second
- A short unit of time; 60 seconds = 1 minute.
- Minute
- 60 minutes = 1 hour.
- Duration
- How long an event lasts.
- Start time
- When an event begins.
- End time
- When an event finishes.
- 24-hour clock
- A day written from 0000 to 2359.
- Time zone
- A region whose clocks use the same local time.
Seconds, minutes and hours
A second is a short unit of time. The second hand makes one full turn in 60 seconds. There are 60 seconds in 1 minute and 60 minutes in 1 hour.
Break seconds into minutes
60 seconds = 1 minute
75 seconds = 1 minute 15 seconds.
See worked example
How many seconds are in 2 minutes 15 seconds?
- 2 minutes = 2 × 60 = 120 seconds.
- 120 + 15 = 135 seconds.
Answer: 135 seconds
Try together
Write 75 seconds in minutes and seconds.
Reveal the answer
75 = 60 + 15, so 75 seconds = 1 minute 15 seconds.
Use a timeline to find duration, start and end
Mark the given times on a timeline. Divide the timeline at whole-hour marks, then add the lengths of the intervals to find the duration. To find an end time, count forward from the start time by the given duration; to find a start time, count backward from the end time by the given duration.
Use the same timeline three ways
A lesson starts at 11:50 a.m. and ends at 12:20 p.m. How long is it?
See worked example
A lesson starts at 11:50 a.m. and ends at 12:20 p.m. How long does it last?
- 11:50 a.m. to 12:00 p.m. is 10 minutes.
- 12:00 p.m. to 12:20 p.m. is 20 minutes.
- 10 + 20 = 30 minutes.
Answer: 30 minutes
Try together
A film ends at 12:20 p.m. after 30 minutes. When did it start?
Reveal the answer
Count back 20 minutes to 12:00 p.m., then 10 more minutes to 11:50 a.m.
Use the 24-hour clock
Use the table to convert 12-hour times to 24-hour times.
| 12-hour period | Hour digits | Example |
|---|---|---|
| 12:00–12:59 a.m. | Use 00 | 12:25 a.m. → 0025 |
| 1:00–11:59 a.m. | Retain the hour; use a leading zero for 1–9 | 8:10 a.m. → 0810 |
| 12:00–12:59 p.m. | Use 12 | 12:25 p.m. → 1225 |
| 1:00–11:59 p.m. | Add 12 to the hour | 4:25 p.m. → 1625 |
Change to 24-hour time
4 + 12 = 16; the 25 minutes stay the same.
See worked example
A show begins at 4:25 p.m. Write its starting time using the 24-hour clock.
- 4:25 p.m. is after noon.
- Add 12 to the hour: 4 + 12 = 16.
- Keep 25 minutes.
Answer: 1625
Try together
Write 8:10 p.m. in the 24-hour clock.
Reveal the answer
2010; 8 + 12 = 20, and the minutes stay 10.
Time-zone maths facts
At the same moment, countries can show different local times. Japan is 1 hour ahead of Singapore; Thailand is 1 hour behind. Count forward for a country ahead, count back for one behind, and change the day when you cross midnight.
Time-zone facts
Japan is 1 hour ahead of Singapore. Thailand is 1 hour behind Singapore.
- 24 hours = 1 day.
- Ahead: count forward by the time difference.
- Behind: count back by the time difference.
- Pass midnight: change to the next or previous day.
It is 10:30 a.m. in Singapore. What time is it in Japan?
Japan is 1 hour ahead of Singapore.
See worked example
It is 10:30 a.m. in Singapore. What time is it in Japan?
- Japan is 1 hour ahead of Singapore.
- Count forward 1 hour: 10:30 a.m. in Singapore → 11:30 a.m. in Japan.
Answer: 11:30 a.m.
Try together
It is 11:30 p.m. in Singapore. What time and day is it in Japan?
Reveal the answer
12:30 a.m. the next day; Japan is 1 hour ahead.