Date Calculator

Date Calculator

This works in two directions. Given two dates, it returns the gap between them in days, weeks, months and years. Given one date and a number of days, it returns the resulting date. Both are calculations people habitually attempt on their fingers and get wrong, because months have unequal lengths and February changes size every four years. The direction you need depends on the question: contract terms, notice periods and project deadlines usually run forward from a known date, while age, tenure and elapsed time run between two known dates. The sections below cover the inclusive-versus-exclusive counting question that causes most disputes over deadlines, and why counting in months is genuinely ambiguous in a way that counting in days is not.

How this is calculated

Difference mode converts both dates to a day count and subtracts, which handles leap years and unequal month lengths automatically. Weeks are that figure divided by seven; months and years are derived calendar-aware rather than by dividing days by 30 or 365.

Add-or-subtract mode takes the start date, converts to a day count, applies the offset, and converts back. This is why adding 30 days to 31 January gives 2 March in a common year and 1 March in a leap year — the arithmetic is in days throughout, and the calendar does the rest.

Worked example: from 1 March 2026 to 1 August 2026 is 153 days, or 21 weeks and 6 days, or exactly 5 months. Adding 90 days to 1 March 2026 gives 30 May 2026.

Inclusive or exclusive — the source of most deadline disputes

The difference between two dates counts the nights between them, not the calendar days touched. From Monday to Friday is 4 by this count, though the period spans 5 calendar days. Both answers are defensible; they answer different questions.

Hotel bookings, interest accrual and age all use the exclusive count. Notice periods, jury service and many contractual deadlines use the inclusive one, and legal drafting usually states which explicitly for exactly this reason. If you need the inclusive figure, add one to the result here.

Why counting in months is ambiguous

A month is not a fixed length, so 'three months from 31 December' has no single correct answer. 31 March is the obvious reading, but 'one month from 31 January' cannot be 31 February, and different conventions resolve it as 28 February, 29 February in a leap year, or 3 March.

Contracts and legislation that care about this specify days instead, or state a convention explicitly. Where you have a choice, expressing a period in days removes the ambiguity entirely — which is why the day count is the figure to rely on when precision matters.

How to use the date calculator

Choose your mode. Difference between two dates, or a date plus or minus a number of days.
Enter the dates or offset. For subtraction, enter a negative number of days.
Decide whether you need an inclusive count. The result counts nights between dates. If your deadline counts both endpoints, add one.

Frequently asked questions

Does the count include both end dates?

The difference counts nights between the dates (exclusive of the end date). Add 1 if you need an inclusive count of calendar days.

How many days are there between two dates?

Enter both in difference mode. The count is exclusive of the end date, so Monday to Friday is 4. Leap years are handled automatically, which is the main reason to use a calculator rather than counting by hand across a February.

What date is 90 days from today?

Use the add mode with today's date and 90 days. Note that 90 days is not three months — depending on which months it spans it can land several days either side of the three-month anniversary.

Why does adding one month to 31 January give an odd result?

Because 31 February does not exist, and conventions differ on how to resolve it — 28 February, 29 February in a leap year, or 3 March are all used somewhere. This is why contracts that need precision specify a number of days rather than months.