How to use the date calculator
- Start date — defaults to today.
- Operation — add to go forward in time, subtract to go back.
- Amount and unit — a whole number of days, weeks, months or years.
The result shows the new date with its weekday and how many calendar days away it is, which is useful when you add months or years and want the exact day count.
Days and weeks vs months and years
Days and weeks have fixed lengths, so adding them is exact: a week is always 7 days, and 90 days is 90 days whichever months it crosses. Months and years are calendar units of different lengths — a month can be 28, 29, 30 or 31 days — so the calculator moves along the calendar instead. Adding 1 month to March 15 gives April 15; adding 1 year to June 2, 2026 gives June 2, 2027.
The difference matters for contracts and deadlines. "Within 30 days" and "within one month" are usually different dates: starting January 15, 30 days later is February 14, but one month later is February 15.
The month-end rule
When the start day doesn't exist in the target month, the result moves back to that month's last day:
| Start | Operation | Result |
|---|---|---|
| January 31, 2026 | + 1 month | February 28, 2026 |
| January 31, 2024 | + 1 month | February 29, 2024 (leap year) |
| March 31, 2026 | − 1 month | February 28, 2026 |
| May 31, 2026 | + 1 month | June 30, 2026 |
| February 29, 2024 | + 1 year | February 28, 2025 |
Spreadsheet functions such as EDATE use the same convention. Note that it isn't reversible: January 31 + 1 month is February 28, but February 28 − 1 month is January 28.
Worked examples
- 90 days after October 8, 2026: 23 days finish October, 30 more finish November, 31 more finish December, and the remaining 6 land on Wednesday, January 6, 2027.
- 6 weeks before October 8, 2026: 42 days back is Thursday, August 27, 2026 — the same weekday as the start, as it always is when you move by whole weeks.
- 18 months after August 31, 2025: the target month is February 2027, which has only 28 days, so the result is February 28, 2027.
Common uses
- Deadlines and notice periods — return windows, net-30 or net-60 payment terms, 90-day probation periods.
- Warranties and subscriptions — the end date of a 1-year warranty or a 6-month plan.
- Pregnancy dating — a due date is conventionally 280 days (40 weeks) after the first day of the last period; the due date calculator does this with more detail.
- Counting workdays — this tool adds calendar days. To see how many weekdays fall between two dates, use the days between dates calculator.
Day 0 or day 1?
Adding 1 day to a Monday gives Tuesday, so the start date counts as day 0. If a rule says something happens "on the 10th day, counting today as day 1", add 9 days rather than 10.
Frequently asked questions
What date is 90 days from today?
Leave the start date as today, choose Add, enter 90 and pick Days. The result card shows the date and weekday.
How do I subtract days from a date?
Switch the operation to Subtract. For example, 30 days before March 15, 2026 is February 13, 2026.
What happens if I add one month to January 31?
February has no 31st, so the result is the last day of February: the 28th, or the 29th in a leap year. The same rule applies to any 29th, 30th or 31st.
Does the calculator handle leap years?
Yes. Day counts include February 29 whenever it falls in the range, and adding a year to February 29 gives February 28 in a common year.
Can I add business days only?
No — this calculator adds calendar days. As a rough guide, 5 business days is 7 calendar days when no holidays intervene; check the result with the days between dates calculator, which counts weekdays.
Last updated: October 8, 2026