Time - Measuring Duration and Sequence
Time is the ongoing sequence of events that allows us to measure duration, order events, and plan the future. From reading a timetable to calculating how long a journey takes, understanding time measurement is an essential life skill.
Before clocks existed, people tracked time with the sun, water, and fire: ancient Egyptian sundials cast a moving shadow to mark the hours of daylight, while water clocks (used in Babylon, Egypt, and later Greece and China) measured time by how much water dripped from one container into another at a steady rate, letting time be tracked even at night or indoors. The 60-minute hour and 60-second minute both trace back to the ancient Babylonians' base-60 counting system, the same civilisation that gave us the 360-degree circle. Timekeeping precision has come a very long way since then: the modern second is no longer defined by the sun or a dripping clock at all, but since 1967 has been defined as a fixed number of vibrations of a caesium atom, measured by atomic clocks accurate to within about one second every hundred million years.
Units of Time
| Unit | Relation to smaller unit | Notes |
|---|---|---|
| Second (s) | Base SI unit | Shortest common unit |
| Minute (min) | 1 min = 60 s | |
| Hour (h) | 1 h = 60 min = 3,600 s | |
| Day | 1 day = 24 h | Earth rotation |
| Week | 1 week = 7 days | |
| Month | 28, 29, 30 or 31 days | Varies by month |
| Year | 365 days (366 in a leap year) | Earth orbit |
The 12-Hour and 24-Hour Clock
The 12-hour clock uses am (midnight to noon) and pm (noon to midnight).
The 24-hour clock runs from 00:00 (midnight) to 23:59 and avoids ambiguity. It is used in transport, the military, and medicine.
| 12-hour | 24-hour |
|---|---|
| 12:00 am (midnight) | 00:00 |
| 6:30 am | 06:30 |
| 12:00 pm (noon) | 12:00 |
| 3:45 pm | 15:45 |
| 11:59 pm | 23:59 |
Converting Between Time Units
| Conversion | Operation | Example |
|---|---|---|
| Hours to minutes | × 60 | 2.5 h = 150 min |
| Minutes to hours | ÷ 60 | 90 min = 1.5 h |
| Minutes to seconds | × 60 | 5 min = 300 s |
| Hours to seconds | × 3,600 | 3 h = 10,800 s |
Worked Examples
3 × 60 + 40 = 180 + 40 = 220 minutes.
14:25 + 2:15 = 16:40.
7 + 12 = 19. Answer: 19:30.
From 09:15 to 11:03 = 1 h 48 min. Answer: 1 hour 48 minutes.
Key Takeaways
- 1 min = 60 s | 1 h = 60 min | 1 day = 24 h.
- 24-hour clock: add 12 to pm hours; midnight = 00:00, noon = 12:00.
- To find a time interval, count on from the start time to the end time.
Practice: Converting Time
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