Time Duration Calculator
How it works
Each duration is turned into a single count of seconds — hours × 3600 plus minutes × 60 plus seconds — before anything is added or subtracted. That single number is then split back into hours, minutes and seconds, which is what normalizes an entry like "90 minutes" into "1h 30m": there is only one way to write a given number of seconds as a clock reading, so the answer comes out normalized whether or not the inputs were.
Subtracting a larger duration from a smaller one gives a negative number of total seconds. Rather than erroring or clamping to zero, the calculator reports the magnitude of the shortfall — how much time short of zero you are — with a minus sign in front, since "you're 30 minutes short" is a more useful answer than a refusal.
Frequently asked questions
Why does 90 minutes become 1h 30m?
The calculator adds and subtracts everything in whole seconds, then splits that single number back into hours, minutes and seconds — so an "unnormalized" input like 90 minutes comes out the same as if you had typed 1 hour 30 minutes in the first place. There is only one clock reading for any given number of seconds, and this is it.
Can the result be negative?
Yes — subtracting a larger duration from a smaller one (an hour minus an hour and a half, say) gives 30 minutes short of zero, shown with a leading minus sign rather than as an error, since it is a perfectly real answer to "how much time is left".
What's this useful for?
Timesheets and billing (adding up hours worked), cooking or task time budgets (does this fit before the next thing starts), and video or audio edit lengths (do these clips add up to the runtime you need).