About this tool
The Time Duration Calculator is a free, browser-based tool that adds or subtracts two time spans and returns a normalized HH:MM:SS value alongside the equivalent total number of seconds.
Enter each duration as HH:MM:SS, MM:SS, or a plain seconds count, then choose add or subtract. Each field is read into a raw second count, combined, and re-expanded into hours, minutes, and seconds — so any part may exceed the usual 0-59 range and is carried over automatically, turning 90:00 into 01:30:00.
The result is zero-padded and paired with a total-seconds figure formatted with thousands separators; subtracting a longer span yields a negative value marked with a leading minus. Everything runs entirely in your browser — no uploads, no accounts, no tracking — and recalculates the moment you edit a field, so it also works offline.