Quick answer
Subtract matching units. If the top minutes are smaller, borrow one hour and add 60 minutes to the top row. If the result is negative, subtract the smaller duration from the larger and put one minus sign in front of the result.
Step by step
- Compare the complete durations to determine the sign. For hand calculation, place the larger duration on top.
- Work right to left. Borrow one minute as 60 seconds or one hour as 60 minutes whenever a column would go negative.
- Subtract each column and attach the sign from step one. Converting both inputs to seconds is a quick way to check the result.
Worked examples
Borrow one hour
8:15 − 3:40 becomes 7 hours 75 minutes minus 3 hours 40 minutes. The result is 4:35.
Keep the negative sign
2:10 − 3:00 is 50 minutes below zero, written −0:50.
Borrow through a zero
1:00:10 − 0:00:45 becomes 0 hours, 59 minutes and 70 seconds minus 45 seconds. The result is 0:59:25.
What to check
Add the amount you subtracted back to the signed answer. You should get the starting duration. For two clock readings, use the elapsed-time method and decide which day the end is on.
Related guides
Sources
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