Find the duration between two times of day, including overnight spans.
Time arithmetic trips people because it is base 60 for minutes and seconds but base 24 for hours, so ordinary decimal intuition fails. Ninety minutes is 1.5 hours as a decimal but 1:30 in clock notation, and mixing the two is the single most common error — adding 1.30 and 1.45 as decimals gives 2.75, while adding 1:30 and 1:45 as times gives 3:15. They are not the same quantity.
The reliable method is to convert everything to a single small unit, do the arithmetic, then convert back. Total the minutes, then divide by 60 for hours and take the remainder as minutes. Two further cases need care: a span crossing midnight needs 24 hours added to the end time before subtracting, or you get a negative result; and payroll conventions often round to the nearest quarter-hour, so a calculated duration and a paid duration can legitimately differ. Where a span crosses a daylight-saving change, the elapsed clock time and the true elapsed time differ by an hour.
Because clock notation is base 60, not base 100. In 1:45 the 45 means forty-five sixtieths of an hour, not forty-five hundredths. Convert to decimal hours or to total minutes before adding, then convert back.
Add 24 hours to the end time before subtracting, or the result is negative. A shift from 22:00 to 06:00 is 30:00 minus 22:00, which is eight hours.
Divide by 60. Ninety minutes is 1.5 hours, and 20 minutes is 0.333 hours. Payroll systems generally want decimal hours, while schedules are usually written in clock notation.
Often rounding. Many payroll systems round each entry to the nearest quarter-hour, so summing exact durations and summing rounded entries produce slightly different totals.