How the arithmetic works
Adding or subtracting times is awkward by hand because the units do not use base ten — 60 seconds make a minute and 60 minutes make an hour. To sidestep that, the calculator converts each duration entirely into seconds, performs the chosen operation on those plain numbers, then converts the total back into hours, minutes and seconds.
Working in a single unit means there is never any "borrowing" to get wrong. The final breakdown is rebuilt cleanly: hours are the whole part of total seconds divided by 3,600, minutes are what remains divided by 60, and seconds are the leftover.
Reading the results
The headline shows the answer in H:M:S clock form. Subtraction can produce a negative duration — for instance taking a longer time from a shorter one — and that is shown with a leading minus sign rather than as an error, which is useful for working out time deficits or how far short of a target you are.
The total hours, minutes and seconds rows express the same result in one unit each, handy when you need a single number to feed into another calculation or a pay rate.
Tips
A couple of pointers:
- You can enter values larger than 59 in a single field (for example 90 minutes); they roll up correctly into the total.
- To add several durations, chain the operations: compute the first sum, then re-enter it as the first duration.
- For wall-clock times of day or shift lengths, the hours and time card calculators are a better fit than this duration tool.
Formula
total = (h₁·3600 + m₁·60 + s₁) ± (h₂·3600 + m₂·60 + s₂)