Time duration calculator

How to use it

1. Pick a calculation. «Between two moments» measures a span; «Add up durations» totals a list of times; «Add to a time» moves a clock time forward or back.
2. Enter dates, times and durations. Times take seconds. In the sum, each line has hours, minutes and seconds plus a button that switches it between adding and subtracting; add or remove lines as you need.
3. Read and copy the result. The result updates as you type, in h:mm:ss and in words, with decimal hours ready for a spreadsheet.

How time arithmetic works

Hours, minutes and seconds are not decimal numbers: an hour has 60 minutes and a minute has 60 seconds. The safest way to do the sums is to turn everything into seconds, add or subtract, then convert back. That is exactly what this calculator does, using whole numbers, so there is no hidden rounding.

To go from seconds to h:mm:ss, divide by 3,600 for the hours, divide what is left by 60 for the minutes, and the remainder is the seconds. Hours do not reset after 24: a 33-hour span is written 33:15:30, which in words is 1 day, 9 hours, 15 minutes and 30 seconds.

When adding up, minutes and seconds above 59 carry over by themselves: 90 minutes is 1 hour 30. If you subtract more than you add, the total turns negative and is shown with a minus sign.

Clock time and daylight saving time

The calculator measures clock time: it treats every day as 24 hours long and does not know your time zone. That is the right answer almost always, with one exception: the night the clocks change.

In most of the United States daylight saving time ends on the first Sunday in November, when 2:00 a.m. becomes 1:00 a.m. again. A shift from 22:00 on October 31, 2026 to 06:00 on November 1 is 8:00:00 on the clock, but 9 hours are actually worked. In spring, on the second Sunday in March, the opposite happens and an hour is lost. If your span covers one of those nights, add or take away an hour yourself.

For a single span within one day, or to turn 7:45 into 7.75, the decimal hours converter is quicker. For a timesheet with several days and breaks, use the work hours calculator; to add days or months to a date, use the date calculator.

Worked examples

What it is useful for

• Travel and deliveries: how long a trip lasts when it leaves one day and arrives the next, or what time you get in after leaving at 23:30 on a 7-hour journey.
• Sports and training: add up lap or stage times with seconds, or find the gap between two finishing times.
• Audio and video: total the running time of a playlist or of the clips in an edit.
• Projects and billing: add up the time spent on a task over several sessions and turn it into decimal hours for an invoice.

Sources

• Hour, minute and second: BIPM, «The International System of Units (SI)», 9th edition, table of non-SI units accepted for use with the SI.
• U.S. daylight saving time: 15 U.S.C. § 260a, as amended by the Energy Policy Act of 2005 (second Sunday in March to first Sunday in November).
• Summer time in the European Union: Directive 2000/84/EC (last Sunday in March to last Sunday in October, at 01:00 UTC).

Sources checked on 14/10/2026.

Frequently asked questions