Method guide
How to Add and Subtract Time: Hours, Minutes, and Seconds
Convert durations to one exact unit, perform the arithmetic, then normalize the signed result back into days, hours, minutes, and seconds.
Quick answer
Convert every duration to seconds, add or subtract those integer-second totals, then divide the absolute result by 86,400, 3,600, and 60 to recover normalized days, hours, minutes, and seconds. If the total is negative, apply one minus sign to the complete duration.
Why time arithmetic is base 60
One minute is 60 seconds and one hour is 60 minutes, so ordinary decimal carrying does not work directly between seconds, minutes, and hours. Converting to seconds creates one exact arithmetic unit.
Convert a duration to seconds
duration seconds = days x 86,400 + hours x 3,600 + minutes x 60 + seconds. For example, 2h 45m 30s is 9,930 seconds.
Add durations
2h 45m 30s is 9,930 seconds and 1h 30m 45s is 5,445 seconds. Their sum is 15,375 seconds, which normalizes to 4h 16m 15s.
Subtract durations
1h 10m is 4,200 seconds. Subtracting 2h 45m, or 9,900 seconds, produces -5,700 seconds. The normalized result is -1h 35m.
Normalize values above 59
You do not need to pre-carry the fields. 90 minutes plus 75 seconds becomes 5,475 seconds, then normalizes to 1h 31m 15s.
Shift a clock time across midnight
Convert the start clock to seconds after midnight, add or subtract the duration, and separate the result into a day offset plus a clock second from 0 through 86,399. 23:30:15 plus 2h 45m 50s becomes 02:16:05 on the next day.
Decimal hours vs. hours and minutes
Decimal hours are the signed total seconds divided by 3,600. They are not a base-60 clock display. For example, 1h 30m is 1.5 decimal hours, not 1.30 hours.
A duration day is not calendar arithmetic
This calculator defines one day as exactly 86,400 seconds. Calendar dates, time zones, daylight-saving changes, and civil-time transitions are different jobs and are intentionally outside this app.