Date Calculator - Add or Subtract Days, Weeks & Months

About the Date Calculator

What date is 90 days from today? When does a six-month contract end? What was the date two years and three months ago? When is a 30-day notice period up, or a 12-week pregnancy scan due? Counting forward or backward on a calendar by hand is slow and easy to get wrong, especially across month ends and leap years.

This date calculator adds or subtracts any combination of years, months, weeks and days from a start date. It gives the resulting date and weekday, the ISO date (year-month-day), and the number of days between the two dates. It follows the standard end-of-month rule when adding months to dates like 31 January, and it tells you when that rule has been applied.

How to Use the Date Calculator

Enter the start date.

Choose add or subtract.

Enter any number of years, months, weeks and days. Leave the others at zero.

The resulting date appears below.

How the Calculation Works

  1. Add (or subtract) the years and months as calendar steps
       29 September 2026 + 2 months = 29 November 2026
  2. Add (or subtract) the weeks and days as exact day counts
       29 November 2026 + 10 days = 9 December 2026

  If the day does not exist in the target month, use the month's last day:
       31 January + 1 month = 28 February (29 in a leap year)

Years and months are calendar units whose length varies; weeks and days are fixed lengths. Applying calendar units first is the convention used by most date libraries and spreadsheets.

Step-by-Step Examples

90 days after 29 September 2026.

  1 day left in September → 30 September
  31 days of October      → 31 October   (32 used)
  30 days of November     → 30 November  (62 used)
  28 more days            → 28 December  (90 used)

The result is Monday 28 December 2026.

6 weeks after 29 September 2026.

  6 × 7 = 42 days → Tuesday 10 November 2026

Adding whole weeks always lands on the same weekday.

1 year, 2 months and 10 days before 29 September 2026.

  − 1 year 2 months → 29 July 2025
  − 10 days         → 19 July 2025   (437 days earlier)

The End-of-Month Rule

Months have 28, 29, 30 or 31 days, so adding a month to a date near the end of a month can land on a day that does not exist. The universal convention is to use the last day of the target month instead: 31 January plus one month is 28 February (or 29 February in a leap year), and 31 March plus one month is 30 April. The same applies to years: one year after 29 February 2024 is 28 February 2025. Note that this rule is not reversible — 28 February minus one month is 28 January, not 31 January.

Days Versus Months

"30 days" and "1 month" are not the same. From 1 February, 30 days later is 3 March in a common year, while one month later is 1 March. Contracts, notice periods and warranties usually specify which they mean, and the difference can matter: a "90-day" return window is about three days shorter or longer than a "3-month" one, depending on the months involved. Enter the period exactly as it is written.

Why Order Matters Near Month Ends

Because months and days are applied in a fixed order, adding "1 month and 1 day" is not always the same as adding "1 day and 1 month". Starting from 30 January 2026, one month gives 28 February (the end-of-month rule), and one more day gives 1 March. Adding the day first gives 31 January, and one month after that is again 28 February. Differences like this only appear near the end of a month, but they explain why two tools can disagree by a day. When a precise date matters legally, check which convention the contract or regulation uses.

Counting Backwards

Subtracting works the same way in reverse. It is useful for finding when a period started — the date a 90-day visa stay began, when to post something so it arrives by a deadline, or the latest date to book a flight with a 21-day advance fare. Enter the end date as the start and choose subtract.

Common Uses

People use date arithmetic for deadlines (a 28-day payment term, a 14-day cooling-off period), contracts and subscriptions (a 12-month phone contract, a 6-month probation), health (medication courses, vaccination intervals, pregnancy milestones), travel (visa lengths such as 90 days in a 180-day period), and projects (sprint lengths and milestones in weeks). For counts that should skip weekends or holidays, use a business days calculator instead.

Understanding Your Result

The headline is the resulting date, with its weekday.

The ISO date line gives the date in year-month-day form, useful for spreadsheets.

The days apart line gives the exact number of days between the start and result.

The weekday line repeats the day of the week.

The worth knowing line says whether the end-of-month rule was applied.

When Should You Use This Calculator?

Use it to find deadlines, due dates and expiry dates.

Use it to work out when contracts, trials or notice periods end.

Use it to find a past date, such as when something started.

Use it to plan events a set number of weeks ahead.

Common Mistakes

Treating a month as 30 days. Enter months as months and days as days.

Forgetting the end-of-month rule. 31 January plus a month is not 3 March.

Counting the start day as day one. Adding 1 day to Monday gives Tuesday.

Ignoring weekends and holidays for working-day deadlines. Use a business days calculator.

Mixing up add and subtract. Check the direction before relying on the date.