How to use the Date Calculator
- Pick a start date, or leave it as today.
- Choose Add or Subtract.
- Enter any mix of years, months, weeks and days, or switch to Business days to skip weekends and listed holidays.
- Read the resulting date with its weekday, day of the year and ISO week number.
Features
- Add or subtract years, months, weeks and days in one go
- Business-day mode that skips weekends and your holidays
- Month-end safe: January 31 + 1 month gives the end of February
- Weekday, day of the year and ISO week for the result
- Handles leap years correctly
- Updates instantly as you type
How dates are added
Days and weeks are simple: a week is always 7 days, so adding 2 weeks and 3 days moves forward 17 calendar days. Months and years are trickier because they vary in length. The calculator follows the rule most people and most software use:
- Years and months are applied first, keeping the same day of the month: March 15 plus 2 months is May 15.
- If that day does not exist in the new month, the result moves back to the month's last day. January 31 plus 1 month is February 29 in 2024 and February 28 in 2025.
- Weeks and days are then added as exact numbers of days.
Because of the month-end rule, adding a month and then subtracting one does not always bring you back to where you started: March 31, 2023 minus 1 month is February 28, and February 28 plus 1 month is March 28.
Leap years
A leap year has 366 days, including February 29. Years divisible by 4 are leap years, except century years that are not divisible by 400: 2000 and 2024 are leap years, but 2100 is not. One year after February 29, 2024 is February 28, 2025.
Business days
In business-day mode the calculator steps one day at a time from the start date and counts only Mondays to Fridays, skipping any holidays you list. The start date itself is not counted, so 10 business days after Friday, March 1, 2024 is Friday, March 15, 2024. If you start on a weekend, the first business day counted is the following Monday.
This matches how most "within 10 business days" deadlines are read, but contracts, courts and government agencies can have their own rules, so check the wording that applies to you.
Handy reference
| Period | Calendar days |
|---|---|
| 1 week | 7 |
| 30 days | 4 weeks and 2 days |
| 90 days | 12 weeks and 6 days, roughly 3 months |
| 6 months | 181 to 184 days, depending on the start date |
| 1 year | 365, or 366 if it includes February 29 |
Extra details in the result
Along with the date and weekday, the result shows the day of the year (1 to 366), the ISO week number and the date in ISO format (YYYY-MM-DD). ISO weeks start on Monday, and week 1 is the week that contains the year's first Thursday, so the first days of January can belong to the last week of the previous year. ISO week numbers are common in European business planning and project schedules.
To count the days between two dates instead, use Days Between Dates, or find an exact age with the Age Calculator.
Frequently asked questions
What date is 90 days from today?
Leave the start date as today, choose Add and enter 90 in the Days box. The result shows the date and its weekday instantly.
What happens when I add a month to January 31?
You get the last day of February: the 29th in a leap year and the 28th otherwise. The calculator never jumps into March for a date that does not exist.
Is the start date counted?
No. The start date is day zero, so adding 1 day gives the next day. In business-day mode the first counted day is the next weekday after the start.
How are business days counted?
Only Mondays to Fridays count, and any holidays you list are skipped as well. Weekends are always skipped.
Can I go back in time?
Yes. Choose Subtract to find the date a number of days, weeks, months, years or business days before the start date.