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ToolBoxGenie

Date Calculator

Calculators · Added 15 August 2026

Add or subtract days, weeks, months and years from a date, optionally counting business days only — or switch to countdown mode to see how far away a future date is. Month arithmetic clamps rather than rolling over, so 31 January plus one month is 28 February, and the tool tells you when that has happened.

What do you want to do?
Direction

How to use the date calculator

  1. 1Choose whether you are shifting a date or counting down to one.
  2. 2For shifting: enter the start date, pick add or subtract, and set the amount and unit.
  3. 3Tick the business days box to skip weekends when working in days.
  4. 4For countdown: enter the target date and read the full breakdown from today.
  5. 5Both modes show the weekday, which is often the thing you actually needed.

Examples

A 90-day deadline

Input
15 August 2026 plus 90 days
Result
13 November 2026, a Friday

Contract and notice periods are usually counted in calendar days unless stated otherwise.

Month-end clamping

Input
31 January plus 1 month
Result
28 February — clamped, because February has no 31st

The tool flags this, because subtracting a month again returns 28 January, not the 31st.

Business days only

Input
Friday plus 5 business days
Result
The following Friday

Weekends are skipped. Public holidays are not, since they vary by country.

About the date calculator

Why month arithmetic is genuinely ambiguous

Adding days to a date is unambiguous — days are all the same length as far as the calendar is concerned. Adding months is not, because months have different lengths and the operation has to decide what to do when the target month is too short.

Two conventions exist. Clamping caps the day at the last valid one, so 31 January plus a month is 28 February. Overflow rolls into the next month, giving 3 March. Nearly all software clamps, because it keeps the result inside the month you asked for, which is what people mean when they say 'a month later'.

Clamping has an uncomfortable property that is worth understanding rather than being surprised by: it is not invertible. Add a month to 31 January and subtract one from the result and you land on 28 January. Add a month to 28, 29, 30 and 31 January and three of them land on the same day. For contractual dates this matters, which is why well-drafted agreements often specify a day of month explicitly rather than saying 'one month later'.

Leap years and the rules people misremember

The Gregorian rule is that a year is a leap year if divisible by four, except centuries, except centuries divisible by 400. So 1900 was not a leap year and 2000 was. The correction exists because the tropical year is about 365.2422 days rather than 365.25, so adding a day every four years overcorrects slightly.

The commonly forgotten part is the century exception, and it has caused real software failures. A number of systems have assumed the simple divisible-by-four rule and produced wrong dates in 1900 or would do in 2100. Spreadsheet software famously carries a deliberate bug treating 1900 as a leap year, retained for compatibility with an even older product.

For date arithmetic in practice, the leap rule mostly surfaces in one place: 29 February. Adding a year to it has to clamp to 28 February in a non-leap year, exactly as month arithmetic does, which is why people born on 29 February have to pick a convention for birthdays in ordinary years. This calculator clamps and reports that it has done so.

Frequently asked questions

Why does 31 January plus one month give 28 February?
Because February has no 31st, so the day is clamped to the last valid day of the target month. The alternative would be rolling over into 3 March, which nearly everyone finds more surprising. Clamping is what spreadsheets, databases and human intuition all do. The consequence worth knowing is that the operation is not reversible: subtracting a month from 28 February gives 28 January, not the 31st you started from.
What counts as a business day here?
Monday to Friday. Saturdays and Sundays are skipped; public holidays are not, because they differ by country, by region within a country, and by year. Applying any one nation's holiday calendar would give a wrong answer for most visitors, so none is applied. If holidays matter for your calculation, count them off the result manually.
Should a deadline be counted in calendar days or business days?
It depends entirely on what the agreement says, and the difference is substantial — 30 business days is about six weeks, while 30 calendar days is about four. Contracts, court rules and notice periods each have their own conventions, and many specify 'working days' or 'clear days' with precise definitions. Read the wording rather than assuming; this tool computes either.
How is this different from the date difference calculator?
That one measures the gap between two dates you already have. This one produces a new date from a start date and an interval, and it handles months and years rather than only days — which is where the clamping behaviour becomes relevant. The countdown mode overlaps a little, but it is oriented around a single future date and what day of the week it lands on.
Which timezone does the countdown use?
Your own. 'Today' is read from your device's local date, and everything is counted in whole calendar days from local midnight. If you need to work across timezones, the timezone converter handles that specifically.

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