This works out how much time separates two clock times — the length of a shift, a journey, a booking, a night's sleep. Enter a start and a finish and it returns the interval in hours and minutes, and also as a decimal, which is the form most timesheets and invoices want.
Times that cross midnight are handled properly rather than coming back negative. A shift from 10.00 pm to 6.30 am is eight and a half hours, not a subtraction that fails; the calculator assumes the finish is the next occurrence of that time after the start. It also copes with the confusion around noon and midnight, where 12.00 am is the start of the day and 12.00 pm is the middle of it — a genuine trap, and the reason many British timetables prefer to write midnight and midday out in full.
One British complication is worth flagging. On the last Sunday in March the clocks go forward at 1.00 am and that night is 23 hours long; on the last Sunday in October they go back and the night is 25 hours. A shift running across either changeover is not the length the clock face suggests, and how it is paid is a matter for the employment contract rather than for arithmetic. For plain time-of-day intervals within a single ordinary day, none of this applies.
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