About the Date Calculator
This date calculator finds the date that is a given number of days, weeks, months or years before or after a starting date. Ask it “what date is 90 days from today?”, “what was the date 6 months ago?” or “when is 2 years and 3 months after my start date?” and it returns the exact calendar date along with its weekday, day of the year and ISO week number.
It is useful for deadlines and notice periods, contract and warranty end dates, medication or probation periods, return windows, subscription renewals and trip planning. The quick-reference table also lists the dates 30, 60, 90, 180 and 365 days from your start date, which are the offsets people search for most.
Years and months are added first, then weeks and days. When adding months lands on a day that does not exist (for example 31 January + 1 month), the result is moved back to the last day of that month — 28 or 29 February — which is how most contracts and spreadsheet functions such as Excel’s EDATE behave.
How to use the date calculator
- 1Pick the start date (for example today).
- 2Choose Add to go forward or Subtract to go back.
- 3Enter any combination of years, months, weeks and days.
- 4Read the resulting date, its weekday and week number.
- 5Use the table for quick 30-, 60- and 90-day dates in both directions.
Formula and method
Calendar months are added first so that “1 month after 15 March” is 15 April, whatever the month lengths. If the target month is shorter than the start day, the result is clamped to its last day (31 Aug + 6 months = 28 Feb, or 29 Feb in a leap year).
Weeks and days are then added as exact 24-hour days, so leap years are handled automatically. The ISO week number follows ISO 8601: weeks start on Monday and week 1 is the week containing the year’s first Thursday.
- Months
- Years × 12 + months, added as calendar months
- Days
- Weeks × 7 + days, added as exact days
Worked examples
90 days from 29 September 2026
Adding 90 days to 29 September 2026 — 1 left in September, 31 in October, 30 in November and 28 in December — lands on Monday 28 December 2026, day 362 of the year and ISO week 53.
6 months before 31 August 2027
Six calendar months before 31 August is February, which has no 31st, so the date is clamped to Sunday 28 February 2027 — 184 days earlier.
2 years, 3 months and 2 weeks after 15 January 2026
Two years and three months after 15 January 2026 is 15 April 2028; two more weeks gives Saturday 29 April 2028, which is 835 calendar days after the start (2028 is a leap year).
Frequently asked questions
What date is 90 days from today?+
Enter today as the start date and 90 in the days field. For example, 90 days from 29 September 2026 is Monday 28 December 2026. The table also lists 30, 60, 120 and 180 days ahead.
How are months added when the day does not exist?+
If the start day is past the end of the target month, the result is the last day of that month. 31 January + 1 month is 28 February (29 in leap years), matching Excel’s EDATE function.
Is adding 1 month the same as adding 30 days?+
No. One calendar month moves to the same day number in the next month, which is 28 to 31 days later. If a contract says 30 days, enter 30 days rather than 1 month.
Does the start date count as day 1?+
No. Adding 1 day to Monday gives Tuesday. If your rule counts the start day as day 1 (common for some medical or legal periods), subtract one from the number of days you enter.
How do I add only business days?+
This calculator counts every calendar day. To skip weekends and public holidays, use the business days calculator in add mode, which works like Excel’s WORKDAY function.