Quick answer
A clock time tells you when something happens. A duration tells you how long it lasts. The same digits can represent either, so label the value before calculating.
Step by step
- If you have a start reading plus a length of time, add the duration to the clock and record any day change.
- If you have two clock readings, identify the date or day of each and subtract to find a duration.
- If you have multiple lengths, add them as durations and keep totals above 24 hours.
Worked examples
Duration + duration
2:30 + 1:45 means 4 hours 15 minutes when both values are elapsed lengths.
Clock + duration
2:30 p.m. plus a duration of 1:45 is 4:15 p.m. the same day.
Clock − clock
4:15 p.m. minus 2:30 p.m. on the same day gives 1 hour 45 minutes.
Quick reference
| Written value | What it could mean |
|---|---|
| 27:15 | 27 hours and 15 minutes of runtime |
| 07:15 a.m. | A morning clock reading |
| 7.25 h | 7 hours and 15 minutes in decimal notation |
| 1:30 | A clock reading or a duration. Ask for the intended unit and context. |
What to check
A value such as 27:15 is valid for a 27-hour duration. A 24-hour clock needs a time within the day plus a separate day or date.
Related guides
Sources
See the calculation methodology or report an error.