⏱How long is it between two dates and times?

Try one of these too

A duration looks like the simplest thing on this site and is the one people most often get wrong, because a span between two clock times is not the same as the number the schedule promised. Breaks come out of it, midnight runs through the middle of it, and twice a year one of the hours does not exist.

Why a span rarely equals what the schedule says

The unpaid break. A shift written as eight hours is usually eight and a half on the clock, because the meal period sits inside the span and outside the pay. The calculator gives you elapsed time; the break comes off afterwards, and it comes off the same way every day, which makes it easy to forget when reconciling a month.

The clocks changing. On the spring-forward day one hour does not exist and on the fall-back day one hour happens twice. A span that crosses the switch is genuinely 23 or 25 hours long even though both wall-clock stamps look ordinary. Overnight shifts are where this bites, and it bites in the direction of the employee twice a year in opposite ways.

Rounding at both ends. Clock-in systems round, people round, and a span measured from two rounded stamps carries both errors. If the total matters, take the raw stamps and let the arithmetic round once, at the end.

Minutes into decimal hours

Once the span is settled, the last step is usually turning it into the decimal a timesheet, an invoice or a payroll system will accept — and that is where a quarter of an hour quietly becomes a tenth.

The six-minute row is why legal and consulting time is billed in tenths: six minutes is exactly 0.1 of an hour, so a six-minute unit needs no rounding at all. The fifteen-minute row is why everyone else bills in quarters. Anything else — a 20-minute call, a 50-minute lesson — produces a recurring decimal and a rounding policy, which is a thing worth agreeing in advance rather than discovering on an invoice.

Timing something that is still running

The same tool answers “how long has this been going on” if you put the start in and leave the finish at now: an outage, a bake, a race, a flight delay, a fermentation, a build. The seconds line is the useful one for short spans, the shifts line for long ones — a 76-hour incident is easier to describe as nine and a half working days than as three days and four hours.

For spans measured in whole dates rather than moments, days between dates is the quicker route, and it reports the same span in hours if you want the crossover. If the span is work rather than wall time, the business days calculator gives working days and the hours they represent at an eight-hour day.

The seconds line, and what it does not know

The total in seconds is exact arithmetic on a calendar that defines every day as 86,400 seconds. Real civil time has not always agreed: a handful of days since 1972 have had one second more, inserted to keep clocks in step with the rotation of the Earth. No ordinary calculator, spreadsheet or browser represents them, and for everything short of satellite navigation and timestamped trading that is the right simplification — but it is a simplification, and it is worth knowing which one you are using before quoting a seconds figure in an audit.

For anything measured in days rather than seconds, weeks between dates and age in days do the long end of the same arithmetic, and days from today works forwards from a start you already have.