How to Calculate the Time Between a Start and End Time

The wrong answer people expect for a start time and an end time that match is 24 hours. The right one is zero. Between those two answers sits the whole of duration arithmetic, and it is worth being exact about what the time between two times actually measures: a position on a 24-hour circle, subtracted from another.

A duration needs no dates

The time between two times is a single subtraction, end minus start, in minutes. 09:15 to 17:45 is 8 hours 30 minutes, which is 510 minutes and 8.5 decimal hours. No date is involved and none is needed, which is why a duration question and a date-range question are different problems even when both involve a clock.

The work hours calculator handles it with one row: switch the row on, type the two times, and leave “Unpaid break” empty. The “Hours per day” line shows the duration, and because only one row is enabled, the “Weekly total” shows the same figure. That duplication is expected rather than a fault.

When the end is earlier than the start

An end time before the start time is read as a shift crossing midnight, and it wraps exactly once. 22:00 to 06:00 is 8 hours. 23:30 to 00:30 is 60 minutes, or 1:00. 11:00 to 01:00 is two hours, not minus ten.

The single wrap puts a hard limit on what the page can represent. A shift of 24 hours or more cannot be entered, because there is no way to tell it apart from a short overnight span or from zero. If you are totalling a long shift, split it across rows and accept that the split is a modelling choice rather than a fact about the day.

Zero means zero

10:00 to 10:00 is 0 minutes, shown as 0:00. So is 09:00 to 09:00, and so is 23:59 to 23:59. The calculator does not promote an equal pair to a full day, which is the intuitive mistake but the wrong arithmetic.

Ending exactly at midnight is the wrap case, not the zero case. 20:00 to 00:00 is four hours, because 00:00 sits earlier in the day than 20:00 and the wrap applies. The boundary matters in the input too: 00:00 is a valid time, 24:00 is not, and a field containing 24:00 is rejected rather than read as midnight.

What two clock times cannot express

Seconds do not exist here. Times are entered as HH:MM from 00:00 to 23:59, so a 09:15:30 start is not representable and a 90-second pause cannot be subtracted. The same applies to a single-digit hour: the field wants two digits.

Time zones do not enter a duration between two wall-clock readings, because both move together and the difference is unchanged. Daylight saving is the awkward case: a local day can be 23 or 25 hours at a change, and two clock times alone cannot say which day you meant, so the calculation cannot account for the missing or extra hour.

Leaving the break field empty and typing 0 into it are the same thing: no deduction. There is no third state in which the calculator assumes a break, so a bare duration is always available.

Breaks are a separate field rather than part of the span. If you want the deduction applied, how to calculate work hours with a lunch break covers where the break goes, and the full day formula with both parts is in how to calculate work hours in a day.

To run the subtraction, the Work Hours Calculator takes the two clock times and returns h:mm alongside decimal hours. It runs in your browser, no FreeAIGeneration account required.