Duration Calculator

Add and subtract a list of durations without carrying by hand. Enter each one in hours, minutes and seconds, mark the row as a plus or a minus, and the running total updates as you type — normalised back to h:m:s and also given as decimal hours, total minutes and total seconds. A total that goes below zero is shown with a minus sign rather than wrapping around.

Time arithmetic is done in base sixty, and that is the whole reason a tool is worth having. Sixty seconds becomes a minute, sixty minutes becomes an hour, and a column of figures done by hand carries wrong at exactly those boundaries: 2:45 plus 2:45 is five and a half hours, not 4:90, and the mistake survives a second look because the digits are plausible. Converting everything to seconds, adding, then converting back is what removes the problem, and that is what happens here.

The decimal hours output is the one that goes into payroll and invoicing systems, and it is where the same confusion reappears in a new costume. 7.75 hours is seven hours and forty-five minutes. It is not seven hours and seventy-five of anything. Multiply the fractional part by sixty to get minutes, and if a figure comes back as 7.45 from a system that means seven forty-five, that system is not giving you decimal hours at all and the two must not be mixed in one column.

One conceptual point worth keeping straight: a duration is a length, not a position. This calculator handles lengths, which add and subtract cleanly, are always the same size, and have no calendar attached. The moment you are working with times of day instead — "from 9:15 on Tuesday until 17:30 on Wednesday" — you have a different problem, because that interval crosses a date and possibly a daylight-saving change, and a day in the middle of it may not be twenty-four hours long. Work out each interval as a duration first, then bring the durations here.