Units of time
| Unit | Equals |
|---|---|
| 1 minute | 60 seconds |
| 1 hour | 60 minutes |
| 1 day | 24 hours |
| 1 week | 7 days |
| 1 year | 12 months, 52 weeks, 365 days (366 in a leap year) |
Time doesn’t work in tens! That’s why time calculations need care.
Reading clocks
Analog clock: The short hand shows the hour; the long hand shows minutes. Each number on the clock stands for 5 minutes for the long hand.
Digital clock: Shows hours and minutes, like 3:45.
a.m., p.m. and the 24-hour clock
- a.m. = midnight to just before noon.
- p.m. = noon to just before midnight.
The 24-hour clock keeps counting after noon: 1:00 p.m. = 13:00, 6:30 p.m. = 18:30. Train schedules and airports use it. To convert a p.m. time, add 12 to the hour (except for 12 p.m., which stays 12:00).
| 12-hour | 24-hour |
|---|---|
| 12:00 a.m. (midnight) | 00:00 |
| 7:15 a.m. | 07:15 |
| 12:00 p.m. (noon) | 12:00 |
| 3:40 p.m. | 15:40 |
| 11:59 p.m. | 23:59 |
Elapsed time
Elapsed time is how long something lasts: from start to end.
Method: count on with a timeline
Draw a timeline and jump to friendly times.
Example: A movie starts at 2:35 p.m. and ends at 4:10 p.m.
- 2:35 → 3:00 = 25 minutes
- 3:00 → 4:00 = 1 hour
- 4:00 → 4:10 = 10 minutes
- Total: 1 hour 35 minutes
Crossing noon or midnight
Example: 10:45 a.m. to 1:20 p.m.
- 10:45 → 11:00 = 15 min
- 11:00 → 1:00 p.m. = 2 h
- 1:00 → 1:20 = 20 min
- Total: 2 h 35 min
Finding an end or start time
- End time: start + elapsed. Practice starts at 4:50 p.m. and lasts 1 h 25 min → 4:50 + 1 h = 5:50, + 10 min = 6:00, + 15 min = 6:15 p.m.
- Start time: count backward. A show ends at 8:05 p.m. after 45 min → 7:20 p.m.
Worked examples
Example 1: 9:20 a.m. to 11:50 a.m.?
2 h 30 min.
Example 2: Write 9:25 p.m. in 24-hour time.
9 + 12 = 21 → 21:25.
Example 3: A bus leaves at 07:40 and arrives at 10:15. How long is the trip?
07:40 → 08:00 (20 min) → 10:00 (2 h) → 10:15 (15 min) = 2 h 35 min.
Common mistakes
- Subtracting times like decimals: 4:10 − 2:35 is not 1:75 or 1.75 hours.
- Forgetting that an hour has 60 minutes, not 100.
- Mixing up a.m. and p.m. around noon and midnight.
Try it yourself
- How long is it from 1:15 p.m. to 3:45 p.m.?
- A class starts at 10:50 a.m. and lasts 75 minutes. When does it end?
- Write 16:05 in 12-hour time.
- How many minutes are in 2.5 hours?
Answers: 1) 2 h 30 min 2) 12:05 p.m. 3) 4:05 p.m. 4) 150 minutes
Quick recap
- Elapsed time = time from start to end.
- Use a timeline and jump to friendly times.
- 60 minutes = 1 hour; the 24-hour clock adds 12 to p.m. hours.