Time Duration Calculator

How to use the time duration calculator

Enter a start time, an end time and the unpaid break in minutes, and the tool returns how long the shift actually was. The example loaded runs 09:00 to 17:30 with a 30-minute break, which is 510 gross minutes and 480 paid, or exactly 8 hours.

Times use native inputs, so your device shows them in whatever format you are used to while the value stored underneath stays unambiguous. The break is entered as a number of minutes rather than as a second pair of times, because that is the shape it arrives in on a timesheet or a rota, and because most breaks are recorded as a duration rather than as a window.

The result is given three ways because three different readers need it. Hours and minutes is the form a person understands: 8 hours 0 minutes on the defaults, 7 hours 45 minutes for an 07:30 to 16:00 shift with 45 minutes off.

Decimal hours is the form payroll and invoicing systems want, and it is the only one that multiplies correctly against an hourly rate. The two are easy to confuse in writing, and the confusion is expensive: 8 hours 30 minutes is 8.50 decimal hours, not 8.30, a gap of twelve minutes on a single shift and over an hour across a week. Total minutes sits underneath both as the figure everything else is derived from.

Shifts that cross midnight are therefore handled without a date field, and that behaviour is the point of the tool for anyone working nights, since a night shift is exactly the case where people reach for a calculator.

It also fixes the meaning of the awkward case where the start and end times are identical: 08:00 to 08:00 is read as a full 24 hours rather than zero, because a shift that begins and ends at the same clock time has run all the way around.

Two limits are worth knowing before you rely on the number. First, the tool works in clock time only and has no date attached, so it cannot span more than 24 hours and cannot know about a daylight-saving transition inside your shift. In most of Europe and North America one night a year is 23 hours long and another is 25, and a shift crossing that hour will be paid differently depending on your local rules.

Second, nothing here decides what should be paid. Whether a break is unpaid, whether travel counts, and what overtime rate applies after a given number of hours are all questions for your contract or your national law, not for a subtraction.

Decimal hours, where timesheets lose money

Decimal hours are where timesheets most often lose money, and the arithmetic is worth spelling out. Decimal hours are paid minutes divided by 60, so 15 minutes is 0.25, 20 minutes is 0.33 recurring, 30 minutes is 0.5 and 45 minutes is 0.75.

Writing 8 hours 30 minutes as 8.30 in a payroll field understates the shift by twelve minutes, and repeated across five shifts that is an hour a week. The tool shows both forms side by side for exactly this reason.

The formula, worked line by line

Everything happens in minutes past midnight, which removes every base-60 trap in one step. A clock time becomes a single integer, the subtraction becomes ordinary arithmetic, and only the final result is converted back into hours and minutes. Trying to subtract 09:00 from 17:30 directly by treating the times as decimal numbers is where hand calculations go wrong, because the minutes column carries at 60 rather than at 100 and a spreadsheet formatted as a number will not tell you.

The overnight case is a deliberate reading rather than an error branch. When the end time is at or before the start, 24 hours are added before subtracting, which turns a negative or zero result into the shift the person actually worked.

That single rule covers both night shifts and the identical-times case, and it is why 08:00 to 08:00 returns a full day rather than nothing. The break is deducted last, and the result is floored at zero so a break longer than the shift can never produce negative time.

start minutes = start hour x 60 + start minute
end minutes   = end hour x 60 + end minute
gross minutes = end minutes - start minutes,  or + 1440 when end is at or before start
paid minutes  = max(0, gross minutes - break minutes)
hours = floor(paid minutes / 60),  minutes = paid minutes mod 60
decimal hours = paid minutes / 60

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