Time Span Calculator

What a time span covers that a plain day count misses

A span between two dates is easy to misread because the two stamps carry different amounts of information. A date-only answer says 15 January to 21 March is some number of days, but it cannot tell you whether a shift that starts at 08:30 and ends at 17:45 on the last day counts toward pay, or whether a contract that begins at noon expires before breakfast on the final date. A time span calculator keeps the clock in the problem: it takes a start date with a start time, an end date with an end time, and reports one single gap in every unit the gap can honestly be written in.

That single gap is the point. The same 431.39 days is 1 year 2 months 6 days on the calendar, 61.63 weeks on a project chart, 10,353.25 hours on a timesheet, 621,195 minutes on a stopwatch and 37,271,700 seconds if you are counting heartbeats. None of those is a rounding of the others. They are different denominations of one amount, the way a bank balance is the same money written as dollars, cents or thousands.

The calendar breakdown, and the clamping rule for short months

Counting a span in years, months and days needs one decision the elapsed totals do not: what a month step means when the target month is shorter. This calculator uses the clamping convention that calendars use, which the U.S. Naval Observatory describes for the Gregorian calendar in its leap year FAQ: a step from the 31st lands on the last day of whatever month follows, and a step from 29 February in a leap year lands on 28 February the next year. The day of the month never overflows into the next month.

The second row is the one people argue about. From 31 January 2024, one month clamps to 29 February, the next month step lands on 29 March, and the remaining 2 days bring you to 31 March. The elapsed count for the same stretch is 60 days. Both rows are correct; they answer different questions, which is why the tool shows them side by side instead of averaging them away.

Leap years: why the same dates a year apart can differ by a day

Every span that crosses 29 February picks up an extra day, and the rule behind that day is strict. Per the U.S. Naval Observatory, Gregorian years divisible by 4 are leap years except centurial years not divisible by 400, so 1700, 1800, 1900 and 2100 are common years while 1600, 2000 and 2400 are leap years. That is why the average Gregorian year is 365.2425 days, a figure the same observatory derives from fitting 97 leap days into every 400 years.

The second row is the trap. Ending one day earlier, on 14 March 2024, drops the total from 366 days to 365, because 29 February 2024 still sits inside the shorter span. A lease, a warranty or a visa window that straddles a leap day can gain or lose a day depending on which stamp the contract names, so it is worth checking both endpoints before you promise a date.

Every unit at once: days, weeks, hours, minutes and seconds

The elapsed totals are pure arithmetic on the difference between the two stamps, so they behave like money: converting between them never loses information, it only changes the denomination. One day is 24 hours, one week is 7 days, and the second itself is a fixed quantity, defined by the National Institute of Standards and Technology as 9,192,631,770 periods of the radiation of the cesium-133 atom, which is what makes the chain of conversions exact rather than approximate.

The chain is worth checking by hand once. Dividing 37,271,700 seconds by 86,400 gives 431.39 days, and 621,195 / 60 = 10,353.25 hours, which is the same figure the day count produces when you multiply back. If a spreadsheet disagrees with either of those two lines, the spreadsheet is using a different pair of stamps or a different rounding point.

Notice that the calendar row and the day row disagree on purpose. Fourteen full calendar months plus 6 days and change is not 14 x 30 plus 6, because the months in between are 28, 29, 30 and 31 days long. When a report asks for months, quote the clamped calendar figure; when a system asks for hours, quote the elapsed figure, and never try to make the two match.

Weekdays and business days inside the same span

A span also contains a work calendar, and that count answers questions the day totals cannot. The convention here is simple: every calendar date the span touches counts, both the start date and the end date included, and a date is a working day when it falls Monday to Friday and is not a US federal holiday. The eleven federal holidays are set by law at 5 U.S.C. 6103, which the Office of Personnel Management lists on its federal holidays page, and when one falls on a Saturday or Sunday it is observed on the adjacent weekday, as the National Archives shows for Independence Day 2026, observed Friday 3 July on its 2026 holiday schedule.

The example starts on 15 January 2024, which is itself the Martin Luther King Jr. holiday, and it runs through thirteen holiday observances across two calendar years. If your deadline is measured in working days rather than dates, 297 is the number to plan around, and the gap between 310 and 297 is exactly the holiday calendar doing its work.

Share of a year: three denominators, three answers

Expressing a span as a percentage of a year needs a denominator, and there are at least three honest ones. A common year is 365 days, an average Gregorian year is 365.2425 days once leap days are spread out, and a fiscal year with its own start and end may be 366 days. The difference sounds small and is not, at the scale of contracts and interest.

The last row is the one that settles arguments: a 365 day span is all of a common year and slightly less than all of an average year, because the average year carries a quarter of a leap day that this particular span did not include. Pick the denominator your counterparty uses, state it next to the percentage, and the number will survive review.

Clock time, elapsed time and daylight saving

The two stamps you type are clock readings, and clock readings are not always 24 hours apart from one day to the next. In United States locations that observe daylight saving time, one Sunday in March has only 23 hours on the wall clock and one Sunday in November has 25. This calculator subtracts the two clock readings as written, which is what a calendar, a contract or a timesheet means by the gap. If you need physical elapsed time instead, for example for machinery runtime or laboratory logging, convert both stamps to a single time scale first: the NIST UTC(NIST) time scale page explains how civil time is kept on a uniform atomic second, and the same idea applies to any zone offset you apply by hand.

A practical check: a span from 01:00 to 04:00 on the spring change date is 3 hours on the clock but 2 hours of real time, and the mirror case in November is 3 clock hours and 4 real ones. When the stakes are pay or SLA credits, decide which reading the agreement intends, because the words on the page usually mean the clock reading.

Which number to quote, and how to check it

The fastest way to use the output well is to match the unit to the audience. Lawyers and lease readers care about the calendar breakdown, because clauses say things like within one year. Project planners quote weeks. Payroll and logistics systems store hours or decimal days. Scientists and engineers quote seconds, the SI base unit. One habit catches most errors: recompute the large unit from the small one before you circulate the number. Ten thousand hours divided by 24 is 416 days and two thirds, so if your span also claims 431 days, one of the two numbers came from a different pair of stamps.

• Quote the calendar breakdown when the underlying document speaks in years and months.
• Quote total days when the question is how long, with no calendar involved.
• Quote business days when the deadline is a working day count.
• State the convention you used, clamping included, so the reader can reproduce the figure.