Time Calculator

How Clock-Duration Arithmetic Works

Adding or subtracting times written as hours:minutes:seconds isn't ordinary base-10 arithmetic — it's base-60 arithmetic for the minutes and seconds columns, with hours left in base-10 (or wrapped to a 24-hour clock if you prefer). This calculator converts every entered duration to a single count of total seconds, adds or subtracts those totals, and then converts the result back to H:MM:SS by dividing: seconds remaining after removing full minutes (total % 60), minutes remaining after removing full hours (Math.floor(total / 60) % 60), and whole hours (Math.floor(total / 3600)). Doing the conversion once, in one direction, avoids the classic bugs that come from trying to carry and borrow across the 60-second and 60-minute boundaries by hand.

Subtraction and Borrowing

When subtracting, if the seconds or minutes in the value being subtracted are larger than the corresponding column in the first value, you "borrow" 60 units from the next column up — exactly like borrowing 10 in ordinary column subtraction, just with a base of 60 instead of 10. Because this calculator works in raw total seconds internally, borrowing happens automatically; if the result would go negative (subtracting a longer duration from a shorter one), it is shown as a negative duration rather than wrapping around a clock face.

Duration Math vs. Calendar Time

This tool is for pure durations — stopwatch-style spans of hours, minutes, and seconds with no attached date, like combining lap times or work segments. If you instead need the elapsed time between two specific calendar dates and times (say, 3:00 PM on March 4th to 9:30 AM on March 19th), use the time duration calculator instead. For simple minute-and-second totals on a timesheet, the hours calculator may also be useful.