Add or Subtract Days from a Date

How the new date is found

Days and weeks are pure arithmetic: the start date becomes a serial day number, the calculator adds or subtracts the number of days (seven per week), and the result is turned back into a calendar date. Nothing about month lengths or leap years has to be remembered, because the serial count already includes every day.

Months and years work on the calendar instead, since they are not a fixed number of days. The calculator moves the month (or twelve months per year) and keeps the same day of the month. If that day does not exist in the target month — 31 April, 30 February, 29 February in a common year — it uses the last day of that month and tells you it did so.

days: serial(start) ± n → date
months: (year × 12 + month) ± n, day = min(start day, days in target month)

Worked examples

Example 1: 90 days after an invoice date

Start 2026-10-20, add 90 days: Monday, January 18, 2027. The span crosses the year boundary and three month ends, which the serial count handles without any special steps.

Example 2: one month after 31 January

Start 2026-01-31, add 1 month. February 2026 has 28 days, so the result is clamped to February 28, 2026. Adding one month to 2028-01-31 gives 29 February 2028, because 2028 is a leap year.

Example 3: counting back

Start 2026-03-15, subtract 6 weeks (42 days): Sunday, February 1, 2026. Subtracting 1 year from 2028-02-29 gives 2027-02-28.

How to read the result

The headline is the resulting date. Below it you get the weekday, the exact number of calendar days between the start and result (useful when you added months and want to know how many days that was), and the ISO date in YYYY-MM-DD form for pasting into spreadsheets and forms. The mini calendar shows the whole result month with the date highlighted, so you can see at a glance whether it lands at a weekend.

Why "add a month" is not "add 30 days"

A calendar month is 28 to 31 days long, so the two operations give different answers. From 1 February, adding 30 days reaches 3 March in a common year, while adding a month reaches 1 March. Subscriptions, rent and loan repayments are usually scheduled by calendar month on the same day each month, falling back to the month end for short months. Notice periods, cooling-off periods and medication courses are usually stated in days. Read which one applies before calculating, and use the unit selector to match it.

Month arithmetic is also not reversible in every case. 31 January plus one month is 28 February, but 28 February minus one month is 28 January, not 31 January. The calculator works from the date you give it each time, which is the behaviour used by most spreadsheet and programming date libraries.

Common uses

Typical tasks include finding a payment due date 30, 60 or 90 days from an invoice; working out when a warranty, trial or return window ends; planning a training block of 12 weeks back from a race day; scheduling follow-ups a set number of weeks apart; and finding the date six months before a passport expiry, which some countries require for entry. Project managers use it to sanity-check timelines, and anyone tracking a habit can find the date of day 100.

Limits and assumptions

This tool adds calendar days, so weekends and public holidays are included in the count. If a deadline is defined in business days, count the weekdays separately — the days between dates calculator shows how many weekdays fall between two dates, which helps you adjust. Results must fall between years 1 and 9999, and the amount is limited to one million units to keep results meaningful. Dates before 1582 follow the proleptic Gregorian calendar. For working out how long ago a birth date was in years and months, the age calculator is the better fit.

Counting conventions for deadlines

Whether "30 days from 1 March" means 31 March or 30 March depends on whether day one is the start date itself. This calculator follows the arithmetic convention: adding 1 day to Monday gives Tuesday, so the start date is day zero. Many statutes and contracts use the same rule ("the day of the event is excluded"), but some schemes count the first day as day one, and some extend a deadline that lands on a weekend or holiday to the next working day. If your deadline lands on a Saturday or Sunday, the mini calendar makes it obvious, and you can check whether your rules move it. Payroll and benefit systems sometimes use a 360-day "banker's year" of twelve 30-day months instead; that is a financial convention, not a calendar one, and is not modelled here.