Time Duration Calculator
The time between two clock times, or two durations added and subtracted — reported in h:mm:ss and in decimal hours. A shift of 1:45 is 1.75 hours, not 1.45, and that one conversion is the most expensive arithmetic error in timekeeping.
A span between two clock times
Both answers come back in h:mm:ss and in decimal hours, from the same count of seconds.
On the 24-hour clock. Seconds are optional — leave them at zero for a shift or a meeting.
If this is earlier in the day than the start, set the next field to say the span ends the following day.
A 22:30 to 06:15 night shift is 7:45:00, not −16:15:00. This is the switch that says so.
09:15:00 to 17:45:00 is 8:30:00. On a timesheet or an invoice that is 8.50 hours — the decimal figure, not the clock one.
- Hours, minutes, seconds
- 8:30:00
- Total hours (decimal)
- 8.50
- Total minutes
- 510
- Total seconds
- 30,600
- Span
- 09:15:00 to 17:45:00, same day
- The decimal figure is the clock figure divided by 60, not the minutes written after a point. 1:45 is 1.75 hours; typing 1.45 on a timesheet gives away 18 minutes of every such entry.
- Everything is counted in whole seconds, so the carries are exact: 90 minutes is 1:30:00 and 3,600 seconds is 1:00:00, with no rounding step in between.
- Clock times carry no time zone here, and no daylight saving is applied. A duration measured on a wall clock is what this returns.
Clocks count in sixties, money counts in tens
A clock is a base-60 instrument inherited from Babylonian astronomy. A pay rate is decimal. Every timesheet, invoice, and billing system in the world sits on the boundary between the two, and the conversion in one direction is a division by 60 while the conversion in the other is a multiplication — neither of which is what happens when someone writes the minutes after a decimal point.
total seconds = h × 3600 + m × 60 + s • decimal hours = total seconds ÷ 3600- h, m, s
- Whole hours, minutes, and seconds as entered. Minutes and seconds are not capped at 59
- total seconds
- The single integer everything is derived from, so no two outputs can disagree
- Carry back
- h = ⌊total ÷ 3600⌋, m = ⌊(total mod 3600) ÷ 60⌋, s = total mod 60
- Next day
- An overnight span adds 86,400 seconds before the carry, and only when you say it should
Reducing to whole seconds before anything else is what makes the carries exact. Ninety minutes becomes 5,400 seconds and comes back as 1:30:00; 3,600 seconds becomes 1:00:00; 59 minutes and 60 seconds becomes 1:00:00 rather than 59:60. There is no rounding step anywhere in that path, so there is nowhere for a 1:29:60 to appear. The only division that produces a fraction is the decimal hours figure, and it happens once, at the end.
Clock times here carry no time zone and no date, which is why no daylight-saving adjustment is applied. The hour the clocks go forward is a property of a calendar day in a particular place; a duration measured between two readings of the same wall clock is not affected by it. If you need a span that crosses a DST boundary in a specific zone, work in dates rather than in clock times.
A 09:15 start, a 17:45 finish, and $6.40
Start: 9 × 3600 + 15 × 60 = 33,300 seconds past midnight. Finish: 17 × 3600 + 45 × 60 = 63,900 seconds. The difference is 30,600 seconds. Carry that back: 30,600 ÷ 3600 is 8 with 1,800 left over, and 1,800 ÷ 60 is 30 — so 8:30:00. Divide the same 30,600 by 3,600 without carrying and you get 8.5 decimal hours. Both readings, one integer, no disagreement possible.
Now the mistake. Someone reads 8:30 off the clock and writes 8.30 on the timesheet. At $32 an hour, 8.5 hours is $272.00 and 8.30 hours is $265.60. The gap is $6.40 — the value of the twelve minutes that 0.30 of an hour is short of half of one. Over a fortnight of shifts that end on the half hour it is $64, and nothing about the number on the page looks wrong.
The night-shift version is worse, because two errors compound. 22:30 to 06:15 the following morning is 81,000 seconds to 22,500 seconds, which is −58,500 until you add the 86,400 seconds of a day: 27,900 seconds, or 7:45:00, or 7.75 hours. Written as 7.45 it loses eighteen minutes; forgotten as an overnight span altogether it comes out negative, which is at least visibly wrong rather than quietly so.
What this calculator assumes
- Clock times are wall-clock readings with no time zone and no date attached. No daylight-saving adjustment is made, and none would be meaningful without knowing where and when.
- Whole seconds throughout. Fractions of a second are not accepted and nothing is rounded before the final decimal conversion, so every carry is exact integer arithmetic.
- Durations are not times of day, so nothing wraps at 24 hours. 38:00:00 stays 38:00:00.
- A span whose end is earlier than its start returns a negative answer rather than assuming an overnight shift. The next-day switch is deliberate, because guessing turns a typo into a plausible shift.
- Nothing is deducted for breaks and nothing is multiplied for overtime. Those are pay-policy questions, not arithmetic ones — see the time card calculator for a week of shifts with breaks taken out.
- Leap seconds are ignored, as they are in every civil timekeeping system. An hour here is always 3,600 seconds.
Time duration FAQ
How do I convert minutes to decimal hours?
Divide the minutes by 60. Fifteen minutes is 15 ÷ 60 = 0.25 hours, twenty minutes is 0.3333, thirty is 0.5, and forty-five is 0.75. Only the multiples of three land on an exact two-decimal figure, and only the multiples of six land on an exact tenth of an hour — which is why payroll systems quantise to six-minute increments in the first place. Everything else repeats, and 0.3333 hours is genuinely twenty minutes however uncomfortable it looks on a timesheet.
Is 1:30 the same as 1.30 hours?
No, and the gap is money. 1:30 is one and a half hours, which is 1.5 in decimal. Writing 1.30 claims thirty hundredths of an hour — eighteen minutes — instead of thirty minutes, so it hands back twelve minutes on that entry alone. The two notations coincide only when the minutes are zero. Everywhere else the shortfall is 0.4 minutes for every minute on the clock: six minutes lost at a quarter past, twelve at the half hour, eighteen at a quarter to, and nearly twenty-four just before the hour. This calculator prints both readings so there is nothing to convert by hand.
How do I calculate hours worked across midnight?
Enter the start and end times as they appear on the clock, then set the third field to say the span ends the next day. A shift from 22:30 to 06:15 is 7 hours 45 minutes — 7.75 decimal hours. Without that switch the arithmetic runs backwards and the tool returns −16:15:00, which is the honest answer to the question as asked and a prompt to flip the switch. It will not silently add 24 hours for you, because doing that would turn a genuine typo into a plausible-looking shift.
Can I add more than 24 hours?
Yes. A duration is not a time of day, so nothing here wraps at 24:00:00. Adding 18 hours to 20 hours gives 38:00:00, not 14:00:00, and a weekly total of 41:30:00 stays 41:30:00. Each box accepts up to 9,999 of its unit, so the components can be entered in whatever way you actually recorded them — 150 minutes is as valid as 2 hours 30 minutes and produces the same 2:30:00.
What does 90 minutes come out as?
1:30:00, and 3,600 seconds comes out as 1:00:00. The minutes and seconds boxes deliberately accept values above 59, because that is how people record things: a meeting ran 90 minutes, a render took 4,500 seconds. Everything is converted to a whole number of seconds first and carried once, so the sexagesimal arithmetic is exact integer arithmetic with no rounding step that could leave 1:29:60.
Does this handle breaks, overtime, or a whole week of shifts?
No — it works out one span or one piece of arithmetic at a time. For a week of clock-in and clock-out pairs with unpaid breaks deducted, use the time card calculator. You can also use this tool for the deduction directly: switch to duration arithmetic, put the shift in the first row and the break in the second, and subtract.
Why does my timesheet round to the quarter hour?
Because US federal rules permit it. 29 CFR § 785.48(b) allows an employer to round recorded time to the nearest quarter hour provided the rounding does not, over time, work against the employee — the so-called seven-minute rule, where 7 minutes rounds down and 8 rounds up. That is a policy applied after the duration is known. This calculator gives you the unrounded figure so you can see what the rounding did.
Sources and review notes
- ISO 8601-1:2019 — the standard behind the
hh:mm:sstime of day, the 24-hour clock, and the representation of durations - 29 CFR § 785.48 — the US Department of Labor rule on recording and rounding hours worked, and the source of the quarter-hour seven-minute convention
- NIST Time and Frequency Division — on civil timekeeping, the 24-hour clock, and why leap seconds are not part of duration arithmetic
There is no dataset behind this page to go stale. Sixty seconds have made a minute since Babylon, and the only thing that changes is the pay rate you multiply the decimal figure by.