Date Add / Subtract Calculator
Add or subtract years, months, weeks and days from a date and get the resulting date, its weekday, the ISO date and the number of calendar days in between.
What is a date add / subtract calculator?
This date add / subtract calculator moves a date forward or backward by any mix of years, months, weeks and days, and tells you where you land: the Resulting date written out in full, its Day of the week, the same date in ISO format (YYYY-MM-DD), and the Distance from the start date in calendar days.
Typical uses are a due date "90 days from today", a renewal "6 months after signing", an assignment due "3 weeks from Monday", or working backward to the date 100 days before an event.
Adding days and weeks is plain counting on a calendar. Adding months and years is less obvious, because months have different lengths. This calculator follows one stated rule: it keeps the same day of the month, and if that day does not exist in the new month, it uses the last day of that month instead. So January 31 plus one month is February 28 (February 29 in a leap year), not March 3. When that adjustment happens, the Distance from the start date line says so. Because of this rule, "add 1 month" and "add 30 days" usually give different answers, and the examples below show both.
Dates are read as calendar dates in your own device's time zone, with no time of day attached, so the answer does not drift by a day because of your location or a daylight-saving change.
Formula
The order the calculator uses
- Years and months first. Years are turned into months (1 year = 12 months) and the month moves forward or back by the total. The year changes when needed: December 15, 2026 + 1 month = January 15, 2027.
- Keep the day, or use the month's last day. New day = the smaller of the start day and the number of days in the new month. A 31st becomes the 30th in April, June, September and November, and the 28th or 29th in February.
- Then weeks and days. Add 7 × weeks + days, counting straight through month and year boundaries.
Subtracting runs the same steps in reverse: years and months back first, then weeks and days back.
Distance from the start date = the number of calendar days between the start date and the result. The start date itself is not counted, so adding 1 day gives a distance of 1.
Month lengths
January, March, May, July, August, October and December have 31 days; April, June, September and November have 30; February has 28, or 29 in a leap year. The month-end adjustment in step 2 can only happen when the start day is the 29th, 30th or 31st.
Leap years
In the Gregorian calendar a year divisible by 4 is a leap year, except century years, which are leap years only if they are divisible by 400. So 2000 was a leap year, while 1900 and 2100 are not. 2024 and 2028 are leap years.
A leap day changes day-based answers but not month-based ones. From March 1, 2027, adding 1 year gives March 1, 2028 (366 calendar days later, because February 29, 2028 falls in between), while adding 365 days gives February 29, 2028.
Why 1 month is not 30 days
A month step follows the calendar, so its length in days depends on where you start: anywhere from 28 to 31 days. From October 16, 2026, adding 3 months gives January 16, 2027, which is 92 days later; adding 90 days gives January 14, 2027. If a rule is written in days, enter days; if it is written in months, enter months.
Worked example
Example 1: January 31 plus 1 month, and plus 30 days
- Start date January 31, 2026, add 1 month. The month moves to February, which has 28 days in 2026, so the 31st becomes the 28th.
- Resulting date: Saturday, February 28, 2026. Distance from the start date: 28 calendar days after the start date (the month step landed on February 28, because February has no day 31).
- In a leap year the same step lands on the 29th: January 31, 2028 + 1 month = Tuesday, February 29, 2028, 29 calendar days later.
- Now add 30 days to January 31, 2026 instead. The first 28 days reach February 28 and the last 2 reach Monday, March 2, 2026.
Example 2: 3 months, 2 weeks and 5 days from September 8, 2026
- Months first: September 8 + 3 months = December 8, 2026. December has an 8th, so no adjustment is needed.
- Weeks and days next: 2 × 7 + 5 = 19 days, and December 8 + 19 days = December 27.
- Resulting date: Sunday, December 27, 2026; ISO format 2026-12-27.
- Check the distance by hand: 22 days to finish September, then 31 in October, 30 in November and 27 in December: 22 + 31 + 30 + 27 = 110 calendar days, which matches the calculator.
Example 3: why the order matters
Start on January 30, 2026 and add 1 month and 1 day. The calculator does the month first: February 30 does not exist, so it uses February 28, then adds 1 day to reach Sunday, March 1, 2026 (30 calendar days after the start date). Doing the day first would give January 31, and one month after that is February 28, a different answer. The calculator always uses the months-first order. If your rule needs the other order, run it twice: add the days, copy the ISO date into the Start date, then add the months.
Example 4: subtracting and leap days
- March 31, 2026 − 1 month − 1 day: March 31 − 1 month = February 28 (there is no February 31), then − 1 day = Friday, February 27, 2026, 32 calendar days before the start date.
- February 29, 2024 + 1 year = Friday, February 28, 2025 (365 days), because 2025 has no February 29. Adding 4 years instead lands on Tuesday, February 29, 2028, 1,461 days later.
How to use this calculator
- Enter the Start date. The form starts with today's date.
- Choose Add or Subtract under Operation.
- Type whole numbers into any of Years, Months, Weeks and Days, leaving the rest at 0. Decimals are cut to whole numbers, so for half a year enter 6 months.
- Read the Resulting date, the Day of the week, the ISO format date and the Distance from the start date. If the distance line mentions a month-end date, the start day did not exist in the new month and that month's last day was used.
- To apply steps in a different order, copy the ISO date into the Start date and run the next step.
Deadlines that skip weekends or holidays
The calculator counts calendar days. It does not skip weekends or holidays, and it does not move a result that falls on a weekend. Many deadlines have their own counting rules. For example, Rule 6(a)(1) of the Federal Rules of Civil Procedure, used in U.S. federal civil cases, says that for a period stated in days you exclude the day of the triggering event, count every day including weekends and legal holidays, and include the last day; if the last day is a Saturday, Sunday or legal holiday, the period runs until the end of the next day that is not. Adding days here matches the first parts of that rule, because the start date is not counted. But 30 days after Thursday, October 1, 2026 is Saturday, October 31, 2026, so under that rule you would check the Day of the week and move forward yourself. State courts, contracts, schools and agencies can count differently, so read the rule that applies or ask whoever set the deadline. To count only Monday-to-Friday days between two dates, use the working days calculator.
Common mistakes
- Entering 1 month when a rule says 30 days, or the reverse.
- Counting the start date as day 1. Here, adding 1 day gives the next day.
- Forgetting that a day-based span crossing February 29 ends one date earlier than you might expect.
To measure the gap between two dates you already have, use the date difference calculator. For more on the month-end question, read how to add or subtract days, weeks, months and years from a date.
Sources
Frequently asked questions
What date is one month after January 31?
February 28, or February 29 in a leap year. February has no 31st, so the calculator uses the last day of February rather than spilling into March. The same rule gives March 31 + 1 month = April 30 and May 31 + 1 month = June 30.
Is adding 1 month the same as adding 30 days?
Usually not. A month step keeps the day of the month, so its length runs from 28 to 31 days depending on the start. January 31, 2026 + 1 month is February 28, but + 30 days is March 2. From October 16, 2026, 3 months is 92 days while 90 days ends on January 14, 2027. Use whichever unit your rule is written in.
Does the calculator count the start date?
No. Adding 1 day to October 16 gives October 17, and the Distance from the start date is 1. If your instructions count the start date as day 1, the last day is one day earlier than the calculator's answer, so subtract 1 from the number of days you enter.
In what order are years, months, weeks and days applied?
Years and months first (with the month-end adjustment), then weeks and days. For January 30, 2026 + 1 month + 1 day this gives March 1, 2026. Applying the day first would give February 28. If you need a different order, run the calculation in two steps.
How does it handle leap years and February 29?
February 29 is included in every leap year, so day counts across it are exact. Adding a year to February 29 lands on February 28 in a common year (February 29, 2024 + 1 year = February 28, 2025) and on February 29 again after 4 years. If you need the day-based version instead, enter 365 or 366 days. For more on February 29 anniversaries, see leap-day birthdays explained.
Can it skip weekends and holidays?
No. It adds calendar days, and the Day of the week output tells you if the result lands on a Saturday or Sunday. Whether a deadline then moves to the next business day depends on the rule that set it.
What happens when I subtract months from a month-end date?
The same rule applies going backward. March 31, 2026 − 1 month = February 28, 2026, and March 31, 2028 − 1 month = February 29, 2028, because 2028 is a leap year.
Why is the ISO date shown?
YYYY-MM-DD lists the year, month and day from largest to smallest unit, so it cannot be confused between month-first and day-first styles (03/04 can mean March 4 or April 3) and it sorts correctly as text in spreadsheets and file names.