How this instrument works
The gap between two clock times looks like simple subtraction until the end time is earlier in the day than the start time — a shift that runs from 11:30 PM to 12:15 AM, for instance, can't be solved by just subtracting 23:30 from 00:15, since that gives a negative number. This calculator handles both cases the same way: it converts each time to minutes since midnight, subtracts start from end, and if the result comes out negative, adds a full 24 hours (1,440 minutes) back in, which correctly accounts for the day boundary being crossed.
Working in minutes-since-midnight rather than hours and minutes separately is what makes the arithmetic reliable — 9:45 becomes 9×60+45 = 585 minutes, and 9:00 becomes 540, so the gap is a single subtraction (585−540 = 45) rather than a multi-step borrow-and-carry calculation across two different units. The same underlying method is what payroll systems use to compute a shift length from clock-in and clock-out times.
This calculator only ever adds one day, so it's built for spans up to 24 hours — a single shift, appointment, or overnight stretch — rather than a multi-day duration. For a gap you know runs across several days, use a full date-and-time duration calculator that also accounts for the calendar date, not just the time of day.
- Enter the Start time — hour (0-23) and Start time — minute using 24-hour (military) notation.
- Enter the End time — hour (0-23) and End time — minute the same way.
- Read Minutes between for the result — if the end time is earlier in the day than the start time, the calculator automatically assumes the end time falls on the next day and adds 24 hours before subtracting.
- For a time you'd normally write as PM, convert to 24-hour notation first by adding 12 to any hour from 1 PM to 11 PM (2:30 PM becomes hour 14, minute 30).
Worked example — an overnight shift from 11:30 PM to 12:15 AM
A night-shift employee clocks in at 11:30 PM and clocks out at 12:15 AM the next morning — in 24-hour notation, start = 23:30 and end = 0:15. In minutes since midnight: start = 23×60+30 = 1,410; end = 0×60+15 = 15.
End minus start is 15 − 1,410 = −1,395, which is negative, signalling the end time fell after midnight. Adding 1,440 gives −1,395 + 1,440 = 45 minutes — the correct length of the shift, even though the raw clock numbers went backward.
Compare that to a same-day case with no wraparound: a lunch break from 12:00 to 13:00 is (13×60) − (12×60) = 780 − 720 = 60 minutes, no adjustment needed, since the subtraction never goes negative.
Questions
How does this handle a time span that crosses midnight?
If subtracting start minutes from end minutes produces a negative number, the calculator adds a full 1,440-minute day back in. That's the same as assuming the end time belongs to the day after the start time — correct for any single overnight span up to 24 hours, like an 11 PM to 7 AM shift.
Can I enter times in AM/PM format?
Not directly — the two hour fields expect 24-hour (military) notation, 0 through 23. Convert a PM time by adding 12 to the hour for any time from 1 PM through 11 PM (so 2:30 PM is hour 14, minute 30); AM times from 1 AM to 11 AM keep their normal hour number, and 12 AM (midnight) becomes hour 0.
What's the maximum span this calculator can compute?
Just under 24 hours. Because it only ever adds one 1,440-minute day to correct for a wraparound, a start and end time that are actually more than a day apart will read the same as a shorter same-day-or-overnight gap — for a genuinely multi-day span, use a date-and-time (not time-of-day-only) duration calculator instead.
Does it matter which time I enter as start and which as end?
Yes — the calculator assumes the end time comes chronologically after the start time (on the same day, or the next day if the clock number is smaller). Entering them reversed will still produce a result, but it'll be the wraparound gap rather than the plain same-day gap you probably intended.
Why does 9:00 to 9:45 give 45 and not some other number?
Both times share the same hour, so the calculation reduces to a straightforward minute subtraction: 45 minutes minus 0 minutes is 45. It's a useful sanity check — any two times within the same hour should give you back exactly the difference in their minute fields.