Time Duration Calculator
Add and subtract durations in hours, minutes and seconds, and see the total in every unit — including decimal hours for payroll.
0 hours · 0 minutes · 0 seconds
Not every day is 24 hours long
Measuring midnight to midnight in real elapsed time across 2026:
| Place | Short day | Long day | Shift |
|---|---|---|---|
| New York | 8 March | 1 November | 60 minutes |
| London | 29 March | 25 October | 60 minutes |
| Lord Howe Island | 4 October | 5 April | 30 minutes |
| Tokyo | — | — | none, ever |
Lord Howe Island is the one worth knowing about. Its clocks move by thirty minutes rather than an hour, so it has a 23½-hour day and a 24½-hour day. Any code assuming daylight saving is always exactly one hour is wrong there, and that is a very common assumption.
So “add a day” is not “add 24 hours”
Add 24 hours to midnight on the morning the clocks go forward and you land at 01:00 the next day, not midnight. Add 24 hours when they go back and you land at 23:00 the evening before. Neither is a bug in the arithmetic — both are the wrong operation.
“The same time tomorrow” and “twenty-four hours from now” are different requests, and they differ on four days a year in most of the world. On the other 361 they agree exactly, which is precisely why the bug hides so well.
The rule: durations are made of seconds, calendar steps are made of days, and converting between them needs a time zone and a date. There is no fixed conversion factor, however much it looks like there ought to be.
How to use
- Enter your durations.
- Add or subtract them.
- Read the total in each unit.
- Use decimal hours where payroll expects them.
Frequently asked questions
Why is 7.30 not seven hours thirty minutes?
Because decimal hours divide by 100, not 60. Seven hours thirty minutes is 7.5 in decimal, and 7.30 means seven hours eighteen minutes. Payroll works in decimal hours, and this misreading is a genuine and recurring source of pay disputes.
How do I convert minutes to decimal hours?
Divide by 60. Fifteen minutes is 0.25, twenty is 0.333, forty-five is 0.75. The awkward ones are the thirds — twenty and forty minutes do not produce clean decimals, which is why rounding conventions matter in timesheets.
Why does subtracting times give a strange result?
Usually because the arithmetic crossed midnight, so the end time is numerically smaller than the start. A shift from 22:00 to 06:00 is eight hours, and naive subtraction gives minus sixteen. Handling the day boundary is what most manual timesheet errors come down to.
Should I round time entries?
Only if your organisation's policy says so, and consistently. Rounding to increments is permitted in some jurisdictions provided it does not systematically favour one side — rounding that always shortens paid time is unlawful in many places.
How do I sum a long list of durations?
Convert everything to a single unit first, usually minutes or seconds, sum it, then convert back once. Adding hours and minutes column-wise and carrying by hand is where errors creep in, and the single-unit approach removes the carry entirely.
What about durations longer than a day?
They are simply larger totals — 30 hours is a valid duration even though it is not a valid time of day. Confusing the two is a common bug in software, where a duration is stored in a field meant for a clock time and wraps around.
🔒 This tool runs entirely in your browser. Nothing you enter is uploaded, logged, or stored.