Time duration calculator
Time does not add in tens, which is why 1:45 plus 0:30 trips people up. This handles the carrying.
Base 60, and the decimal column
Minutes and seconds roll over at 60 rather than 100, so adding durations in your head means carrying at an unfamiliar boundary. Subtracting is worse, because borrowing takes 60 rather than 10.
The decimal hours row is the one worth knowing about. Payroll systems, invoices and project trackers almost always want hours as a decimal, and the conversion is a common source of quiet errors: 1 hour 30 minutes is 1.5 hours, but 1 hour 15 minutes is 1.25, not 1.15. Reading the wrong one off a timesheet under-reports or over-reports every single entry.
Subtracting a longer duration from a shorter one gives a negative result rather than an error, since that is usually what you meant — how far under or over a target you landed.
Questions
How do I convert minutes to decimal hours?
Divide by 60. Fifteen minutes is 0.25 hours, twenty is 0.333, forty-five is 0.75. The decimal row above does it for you, which avoids the classic mistake of writing 1.15 for an hour and a quarter.
Can I add more than two durations?
Add the first two, then put that result back into the first row and add the next. The running total carries correctly each time.
What about durations over 24 hours?
They work normally — the hours field is not capped at 24, so a 40-hour week adds without issue.