Time Duration Calculator

A time duration calculator measures the length of time between two clock times and returns it in hours, minutes, and seconds. Enter a start time and an end time, and it tells you exactly how long the span is, correctly handling the case where the end time falls after midnight so an overnight shift from 10:00 PM to 6:00 AM reads as eight hours rather than a negative number.

It does more than subtract two times. A second mode adds or subtracts a duration from a starting time, so you can answer questions like what time is it 2 hours and 45 minutes after 3:30 PM, and it tells you when the result rolls into the next day. A third mode totals a list of durations, which is how timesheets, study logs, and billable-hours records get summed without carrying minutes by hand.

Every result is shown three ways: as a clock value like 08:00:00, as a plain summary like 8h 0m, and as decimal hours like 8.0, because payroll and invoicing usually want the decimal form while schedules want the clock form. All of it runs entirely in your browser, so nothing you enter is uploaded or stored, and the tool keeps working offline once the page has loaded.

Spans that cross midnight are the usual source of a wrong answer, so a finish time earlier than the start is treated as the next day rather than as a negative duration. That is what you want for a shift ending at 02:00 and wrong for a data-entry mistake, so check the total looks plausible. Decimal output exists because payroll systems want hours as a number rather than as hours and minutes, and the two disagree in the last digit: 7 hours 20 minutes is 7.33 hours, and a week of those totals slightly differently depending on which form you add up. Round once at the end rather than per entry if the figures are going on a timesheet.