How this calculator works
Each duration is turned into seconds, then the chosen arithmetic runs on those whole numbers. In difference mode the second duration is subtracted from the first. In the other two modes the same relation, first minus second equals difference, is rearranged to solve for the missing value. The page does not borrow 60 seconds or 60 minutes during the subtraction, so no placeholder mistakes are possible.
Formula
seconds = 3600H + 60M + S for each entry. Difference mode: D = S1 - S2. Solve for the second: S2 = S1 - D. Solve for the first: S1 = S2 + D. A negative difference is shown with a minus sign and means the second duration is longer.
Worked example
5:30 minus 1:45 is 3:45, or 13500 seconds. If the first duration is 5:30 and the difference is 3:45, the second is 1:45. If the second is 1:45 and the difference is 3:45, the first is 5:30.
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Frequently asked questions
Why can the difference be negative?
In difference mode the order you entered the durations matters. A negative answer simply means the second duration is longer than the first. The clock text carries the minus sign so the direction is still visible.
Why does solving for a duration reject a negative result?
A duration is a length of time, so a negative value would not be meaningful. If the numbers would produce one, the page asks you to check which value is the difference.
Does this work across midnight or between dates?
No. These are abstract durations with no date attached. To measure real elapsed time across midnight or a clock change, use the time duration calculator with dated readings.