Question 1

Can I add more than two durations?

Accepted Answer

This calculator adds or subtracts exactly two durations at a time. To sum three or more, chain the results: add the first two, then take that result and add the third. For large sets of durations (e.g. a full week of time entries), a spreadsheet with SUM() is more efficient.

Question 2

What is decimal hours and why does it matter?

Accepted Answer

Decimal hours express a duration as a single number (e.g. 1.5 instead of 1:30). Payroll systems and invoicing software require decimal hours because they can be directly multiplied by an hourly rate. For example, 2.25 hours × $60/hr = $135.

Question 3

What if my subtraction result goes negative?

Accepted Answer

If Duration 2 is longer than Duration 1 when subtracting, the result would be negative — which isn't meaningful for time durations. The calculator will show an error and ask you to swap the durations or switch to addition.

Question 4

How does carry-over work when adding times?

Accepted Answer

By converting everything to seconds first, carry-overs happen automatically. 45 minutes + 25 minutes becomes 70 minutes, which correctly becomes 1 hour and 10 minutes in the output — no manual carrying required.