Duration & elapsed time
Theory
To find how long something lasts, count on: first to the next o'clock, then in whole hours, then the extra minutes. An hour is \(60\) minutes, so never subtract times like decimals.
Duration, or elapsed time, is how long something lasts from a start time to a finish time.
The safest way to find it is to count on: jump from the start to the next o'clock, then in whole hours, then on to the finish, and add the jumps.
An hour is \(60\) minutes, so times cannot be subtracted like ordinary numbers. If both times are in the same hour, just take the minutes away.
Break the journey at each o'clock.
| Jump | Minutes |
|---|---|
| 10:40 to 11:00 | \(20\) |
| 11:00 to 12:00 | \(60\) |
| 12:00 to 12:15 | \(15\) |
| Total | \(95\) |
\(95\) minutes is the same as \(1\) hour \(35\) minutes, because \(95 - 60 = 35\).
Counting on to find a duration
- Jump to the next o'clock and note the minutes.
- Jump in whole hours up to the last o'clock before the finish.
- Jump the extra minutes on to the finish time.
- Add the jumps. Convert to hours and minutes if needed, remembering \(60\) minutes in an hour.
Both times are in the same hour, so subtract the minutes.
| \(50 - 15\) | \(=\) | \(35\) |
It lasts \(35\) minutes.
Count on to 9:00, then in whole hours.
| \(40 + 120\) | \(=\) | \(160\) |
That is \(2\) h \(40\) min.
Jump \(+20\), \(+60\), then \(+15\).
| \(20 + 60 + 15\) | \(=\) | \(95\) |
It is \(95\) minutes.
Count on \(15\) to 8:00, leaving \(80\) minutes, which is \(1\) h \(20\).
| \(8{:}00 + 1\text{ h }20\) | \(=\) | \(9{:}20\) |
It finishes at 9:20 pm.
Common pitfalls
Frequently asked questions
How do I work out elapsed time?
Count on from the start time: first to the next o'clock, then in whole hours, then the extra minutes. Add the jumps for the total.
Why can't I just subtract the two times?
Because an hour is \(60\) minutes, not \(100\). Subtracting like decimals gives the wrong answer, so counting on is safer.
How long is 95 minutes in hours and minutes?
\(1\) hour \(35\) minutes, because \(95 - 60 = 35\).
How do I find a finish time from a duration?
Count on the duration from the start time. A film at 7:45 pm running \(95\) minutes finishes at 9:20 pm.
What if both times are in the same hour?
Just take the minutes away. From 9:15 to 9:50 is \(50 - 15 = 35\) minutes.