Dates calculators

Every dates calculator here follows the same rules: results update live as you type, the formula is shown with a worked example, and the math runs entirely in your browser — nothing you enter is uploaded anywhere. Each one is unit-tested against published reference values before it ships.

Why date arithmetic is harder than it looks

Dates are the one everyday calculation where careful people routinely get different answers, and almost always for the same three reasons. Months are not a fixed length, so "three months from 30 November" has no single obviously correct answer and different systems resolve it differently. Leap years follow a rule most people half-remember: divisible by four, except centuries, except centuries divisible by 400 — which is why 2000 was a leap year and 1900 was not. And the endpoints of a range may or may not count, which is the difference between five days of leave and six.

These calculators use the proleptic Gregorian calendar throughout and state their endpoint convention on each page. That is not pedantry: it is the reason two people counting the same fortnight can disagree, and it is worth knowing which convention your employer or contract uses before you rely on any tool, including this one.

Which calculator to reach for

Use days between dates when you have two fixed dates and want the span — notice periods, contract terms, how long since something happened. Use add or subtract days when you have one date and a duration, which is the shape of most deadline questions. Use the age calculator when you need years, months and days rather than a raw day count, since "37 years, 2 months and 11 days" is the form documents ask for. Use the countdown for a live figure to a future moment, and the hours calculator for a working day with breaks deducted.

Working days, holidays and the limits of a calendar

A calculator knows the calendar; it does not know your country, your public holidays, your company shutdown or whether your contract counts weekends. Public holidays vary by nation and often by province, and several move each year — Easter is the well-known one, and any holiday falling on a Sunday may be observed on the Monday depending on local law. Where a page offers a working-day count it says exactly what it excludes, and that will be weekends only unless stated. For anything with legal or contractual weight, check the raw span here and then apply your own jurisdiction rules on top.

Time zones, and the day that goes missing

These calculators work in whole calendar days in your local time. That is the right model for the questions people actually ask — deadlines, ages, notice periods — but it means a countdown viewed from two time zones can legitimately show different numbers of days remaining, and a daylight-saving transition makes one day in the year 23 hours long and another 25. For most purposes this is invisible. For anything scheduled across zones to the hour, work from an explicit UTC timestamp rather than a day count.