Date Calculator
Date arithmetic without the mistakes
Difference between two dates, or a date shifted by any span — with an option to count working days only.
Date Calculator
Runs in your browser · nothing uploadedBusiness days by month across the span. Bar chart with 4 steps across the term, falls from 100% at the first step, to 100% at the midpoint, to 80.95238095238095% by the final step.
01 — How to use Date Calculator
02 — Getting good results
03 — Questions
No. Everything is calculated in this browser tab, including the holiday rules, which are code rather than a lookup service.
With the anonymous Gregorian algorithm, which derives the date from the year alone. It is exact for any year in the Gregorian calendar, so Good Friday and Easter Monday are correct without a stored table.
Because each one implemented here is computed from its real rules, including weekend substitution, and verified against published dates. Adding more countries means encoding each set of rules properly; a long list of half-right calendars would be worse than a short list of correct ones.
You get 28 February, or 29 in a leap year — the day clamps back to the end of the target month. Without that rule the underlying date object silently rolls forward into March, which is the single most common date-arithmetic bug.
04 — More about Date Calculator
Why date libraries exist
Date arithmetic looks like a solved problem until you enumerate the cases. Months vary in length, so "one month later" needs a rule for days that do not exist in the target month. Years vary in length, so day counts drift. Weeks do not divide years evenly, so week numbering needs its own standard, and ISO 8601 supplies one where the first week of a year can begin in December. Public holidays follow rules rather than dates, and those rules include substitution when a holiday lands on a weekend. Time zones and daylight saving add a further layer in which a "day" is occasionally twenty-three or twenty-five hours long — which is why this tool works in UTC for the day-level arithmetic it does, rather than producing answers that change depending on when you load the page.
Business days and what they assume
Counting working days seems simple: skip Saturdays and Sundays, skip public holidays. Both halves carry assumptions. The weekend is not Saturday and Sunday everywhere — much of the Middle East observes Friday and Saturday, and some countries use a five-and-a-half day week. Public holidays vary not just by country but within it: Scotland and Northern Ireland have bank holidays England does not, and US states differ on which federal holidays they observe. Regional variation, company shutdowns and industry conventions all sit on top. What a business-day count gives you is a reasonable default for planning, not a contractual number; where the count matters legally, the governing document usually defines its own set, and that definition wins over any calculator.
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