Time Duration Adder

Time duration adder

This tool adds and subtracts a list of hours-minutes-seconds durations and gives the combined total in three forms: HH:MM:SS, raw seconds, and decimal hours. It is handy for billing time, editing audio and video, logging training, netting project hours against a budget, or converting between formats.

How it works

Each row has an add/subtract toggle and hours, minutes, seconds fields. The tool converts every row to whole seconds (h × 3600 + m × 60 + s), negating it if the row is set to subtract, then sums all rows. The total seconds are re-expanded into HH:MM:SS, and also shown as a plain second count and as decimal hours:

total seconds = Σ (±(h × 3600 + m × 60 + s))
HH:MM:SS      = expand(total seconds)
decimal hours = total seconds ÷ 3600

All of it is exact integer arithmetic, so there is no rounding loss. Inputs are not capped at 60, so entering 90 minutes normalises correctly to 1 hour 30 minutes in the output.

Worked example — billing multiple tasks

A freelancer spent three blocks on a client project: a 1-hour 15-minute strategy call, a 2-hour 30-minute writing session, and a 45-minute review. They then deduct a 30-minute personal break that crept into that review.

Add    1:15:00   (strategy call)
Add    2:30:00   (writing session)
Add    0:45:00   (review)
Sub    0:30:00   (personal break)

Seconds: 4500 + 9000 + 2700 − 1800 = 14400
Total:   04:00:00  ·  14400 s  ·  4.0 decimal hours

Four decimal hours is what goes on the invoice. Decimal form is what most billing, payroll, and spreadsheet systems expect — it avoids the ambiguity of whether “1:30” means 90 minutes or 1 hour and 30 hundredths.

Common use cases

Audio and video editing. Add clip durations to check an assembly cut against a delivery specification, or subtract a section you are removing to see the new runtime. When a client asks for a 3-minute highlight reel and you have six clips, enter all six and see instantly whether you need to cut or have room to add.

Sports and training logs. Total weekly training sessions, subtract rest periods, and compare against a target weekly volume. For example, three runs of 42 minutes, 38 minutes, and 55 minutes plus a 90-minute bike ride: add all four and the tool returns the weekly total.

Timesheet reconciliation. Net billed hours against unbillable overhead to verify you have accounted for the full working day, and flag any gap before submitting. Many freelancers find a gap of 30–60 minutes per day that was previously unaccounted for.

Podcast and video production. Sum segment durations (intro, interview, ad break, outro) to hit a target episode length, subtracting any segments you decide to cut in editing.

Meeting time audit. Log every meeting in a week, sum them, and see how many hours were consumed by meetings versus focused work. The subtraction feature lets you deduct time that was genuinely productive preparation vs passive attendance.

Why three output formats?

HH:MM:SS is the human-readable form useful for telling someone how long something runs.

Total seconds is useful when passing the value to software, scripts, or spreadsheet formulas that expect a raw number.

Decimal hours is the form used by most billing systems, payroll software, and project management tools — a 4-hour 30-minute block is 4.5 hours in an invoice, not “4:30.”

Negative results are shown with a minus sign if the subtractions exceed the additions — useful for spotting when a cut takes a video below its minimum length, or when remaining budget hours are less than remaining task time.

It is privacy-first: every duration you enter stays in your browser and is never sent anywhere.