📅 Date Calculator

Add or subtract time from any date, or find the difference between two dates.

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📅 Date Calculator

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Related Guide
Date Math: Adding, Subtracting and Counting Days Between Dates
Why month lengths and leap years make date math tricky by hand.
Read Article →

What this date calculator does

There are two modes. Add / Subtract moves a start date forward or back by a number of days, months and years and shows the resulting date and its weekday. Difference counts the days between two dates and also shows the span in weeks, approximate months and approximate years. The sums run in your browser and nothing you type is sent anywhere.

How adding and subtracting works

Years and months are applied first, as whole calendar months, and days are added last. If the start day does not exist in the target month, the result is the last day of that month, which is how spreadsheets and most date libraries behave.

Worked examples

  • 90 days from 6 October 2026 is Monday, 4 January 2027.
  • 31 January 2026 plus 1 month is 28 February 2026, since February has no 31st. Subtracting 1 month from 31 March gives the same day.
  • 29 February 2028 plus 1 year is 28 February 2029, as 2029 is not a leap year.
  • 20 November 2026 plus 3 months and 15 days: 3 months takes you to 20 February 2027, then 15 days gives Sunday, 7 March 2027.

Note that "one month" is not a fixed number of days. Adding 1 month to 15 January moves to 15 February, which is 31 days later, while adding 1 month to 15 February moves to 15 March, which is 28 days later in a normal year. If a contract says "30 days" use days, and if it says "one month" use months.

How the difference is counted

The total is the number of calendar days from the start date to the end date, so 6 October 2026 to 4 January 2027 is 90 days (12.9 weeks). The start date is not counted and the end date is, so Monday to Tuesday is 1 day. If you need both ends counted, for example for a hotel "number of days", add 1. Weekends and public holidays are included. For working days only, use the days between dates calculator. Months are shown as days ÷ 30.44 and years as days ÷ 365.25, which is why they are labelled approximate.

Where date counting goes wrong

  • Notice periods and deadlines. Contracts and local rules differ on whether the first day counts, whether a deadline falling on a weekend moves to the next working day, and whether the week starts on Sunday or Monday. Working weeks and public holidays differ by country, and sometimes by employer. Use the calculator to get the arithmetic and confirm the counting rule in the contract or with an adviser.
  • Time zones. The calculator works with dates only, with no time of day, so it is not affected by time zones or daylight saving.
  • Leap years. A year is a leap year if it is divisible by 4, except century years not divisible by 400, so 2000 and 2028 are leap years and 2100 is not. The calculator handles this automatically.

🔗 Related Calculators

Age Calculator Days Between Dates Time Calculator Hours Calculator Due Date Calculator Hours Calculator

Frequently Asked Questions

Set the start date to today, choose Add, enter 30, 60, or 90 in the Days field, and click Calculate. These intervals are commonly needed for contract deadlines, invoice payment terms, and return windows, and the calculator accounts for the exact number of days in each month.
No, it counts all calendar days between the two dates. For a business-days-only count that excludes weekends (and optionally holidays), use the Days Between Dates calculator instead.
Because months and years don't have a fixed number of days — a month can be 28 to 31 days and a year can be 365 or 366. The calculator converts the exact day count using average lengths (30.44 days/month, 365.25 days/year) to give you a readable estimate alongside the exact day and week totals.