Time Duration Calculator — Add & Subtract HH:MM:SS

How the Time Duration Calculator Works

The calculator has two modes. In Add/Subtract mode, each row is a duration you enter as HH:MM:SS. Toggle the sign button between + and − to add or subtract that duration. The running total updates instantly and is shown as HH:MM:SS, total minutes, and total seconds.

Add rows — click "+ Add duration" for as many entries as you need.
Between Two Times — enter a start and end clock time (HH:MM) and get the exact delta. Midnight crossings are handled automatically.
Save compositions — save up to 5 row combinations to localStorage and reload them later.

Common Use Cases

Video editing is the most frequent need. You have six clips — 3:24, 0:45, 2:10, 1:55, 4:02, and 0:30 — and you need to know if the total edit fits within a 15-minute slot. Add them all up in one view, no mental math required.

Billing by the hour is the other common scenario. You worked 2:15 on project A, 0:45 on a meeting, and 1:30 on revisions. Subtract a 1-hour lunch. The tool gives you the billable total in both HH:MM:SS and decimal hours — useful when your timesheet software wants a decimal.

Athletes and coaches use it too. A swim practice has eight intervals: add them to find the total in-water time, subtract rest periods, and confirm that the session fits the pool booking.

Save and reload compositions

The save button stores your current set of rows in localStorage — up to 5 compositions. Each saved entry shows a compact summary and a load button. Useful if you come back to the same set of durations weekly. The between-two-times mode handles midnight crossing: if end is before start, the tool assumes the next day.

What isn't here: a time-zone-aware duration calculator, recurring schedules, or a Gantt-style visualization. Those are separate tools with separate complexity. This one does one thing: HH:MM:SS arithmetic.

Frequently Asked Questions

How do I add two time durations?

Enter each duration as HH:MM:SS in its own row with a + sign. The running total updates instantly. For example, 1:30:00 + 0:45:30 = 2:15:30.

Can I subtract a duration from a total?

Yes. Click the sign button on any row to toggle it from + to −. The row will be subtracted from the running total. You can mix additions and subtractions freely.

What happens if the total goes negative?

The total displays with a minus sign prefix (e.g., -00:15:00). This can happen if subtractions exceed additions. It's valid output — useful for tracking overtime versus scheduled hours.

How is the 'between two times' mode different?

Instead of adding durations, you enter two clock times (start and end). The tool calculates the delta between them. If the end time is earlier than the start time, it assumes the end is on the following day (midnight crossing). This is useful for overnight shifts or flights.

How do I convert a duration to decimal hours?

The calculator shows the total in decimal hours automatically below the HH:MM:SS result. For example, 1:30:00 = 1.5000 hours. This is useful for billing or timesheet software that expects decimal hours.