Date Calculator
- Updated
- Formula shown
- Free, no sign-up
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How to use the date calculator
- Pick Add or subtract to move from a date, or Difference between dates to measure the gap between two dates.
- Choose calendar days or business days (Monday to Friday).
- Enter the start date (today by default) and the years, months, weeks or days. Blank boxes count as zero.
- Read the result date and its weekday. A note appears if a month-end date had to move to a shorter month.
How dates are added
Step 2: result = step 1 date + (weeks × 7 + days)
Business days: step one day at a time, counting only Monday to Friday
Years and months are calendar units, so they are added first and keep the day of the month: March 15 plus 2 months is May 15, however many days April has. If the target month is too short, as with January 31 plus one month, the date moves to that month's last day instead of spilling into the next month. Weeks and days are fixed lengths and are added afterwards, the same order used by common date libraries.
For a difference, the calculator subtracts the day numbers of the two dates. The years, months and days figure counts whole months from the start date with the same month-end rule, then the days left over, so adding the result back to the start date gives the end date exactly.
Worked examples
- 90 days from Monday, October 19, 2026 is Sunday, January 17, 2027. October has 12 days left after the 19th, November 30 and December 31, which makes 73, and 17 more days reach January 17.
- 10 business days from the same Monday is Monday, November 2, 2026: two full working weeks, or 14 calendar days.
- January 31, 2026 plus 1 month and 1 day is Sunday, March 1, 2026. The month step lands on Feb 28, 2026, because February 2026 has 28 days, and the extra day moves it to March 1.
- From January 1 to December 31, 2026 is 364 days, including 260 weekdays after January 1.
Common deadlines from October 19, 2026
How the same number of days lands differently when you count every day or only weekdays (public holidays not removed).
| Days | Calendar days later | Business days later |
|---|---|---|
| 7 | Mon, Oct 26, 2026 | Wed, Oct 28, 2026 |
| 14 | Mon, Nov 2, 2026 | Fri, Nov 6, 2026 |
| 30 | Wed, Nov 18, 2026 | Mon, Nov 30, 2026 |
| 45 | Thu, Dec 3, 2026 | Mon, Dec 21, 2026 |
| 60 | Fri, Dec 18, 2026 | Mon, Jan 11, 2027 |
| 90 | Sun, Jan 17, 2027 | Mon, Feb 22, 2027 |
| 120 | Tue, Feb 16, 2027 | Mon, Apr 5, 2027 |
| 180 | Sat, Apr 17, 2027 | Mon, Jun 28, 2027 |
Adding months to the 31st
Adding months in one step keeps the original day whenever it exists. Adding one month at a time can drift, because each step starts from the shortened date. Starting from January 31, 2026:
| Months added | In one step | One month at a time |
|---|---|---|
| 1 | Feb 28, 2026 (month end) | Feb 28, 2026 |
| 2 | Mar 31, 2026 | Mar 28, 2026 |
| 3 | Apr 30, 2026 (month end) | Apr 28, 2026 |
| 4 | May 31, 2026 | May 28, 2026 |
| 5 | Jun 30, 2026 (month end) | Jun 28, 2026 |
| 6 | Jul 31, 2026 | Jul 28, 2026 |
Business days and holidays
The business-day option skips Saturdays and Sundays only. Public holidays differ by country, state and even employer, so they are not removed automatically. If a holiday falls inside your period, add one business day for each. Counting forward, a start date on a weekend behaves like the Friday before it, so Saturday plus 1 business day is Monday; counting backward, it behaves like the Monday after it. When comparing two dates, the count includes the end date but not the start date, the same rule as adding, so the two modes always agree.
Tips and common mistakes
- "Within 30 days" vs "1 month". They are not the same: from January 31, 30 days is March 2 in 2026, while 1 month is February 28.
- Check whether the first day counts. Legal and contract deadlines often say whether the day of the event is day 0 or day 1. This calculator treats the start date as day 0.
- Subtracting works the same way. March 31 minus 1 month is the last day of February, not March 3.
- Historic dates use today's Gregorian calendar extended backwards, which is the standard for date arithmetic.
For a plain count of days with options to include the end date or skip weekends, use the day counter. To include times of day, try the time duration calculator, and for ages the age calculator.
Frequently asked questions
What date is 90 days from today?
Enter today's date (the default), choose Add and type 90 in Days. From Monday, October 19, 2026, for example, 90 days later is Sunday, January 17, 2027. 90 business days from the same Monday is Monday, February 22, 2027, because weekends are skipped.
What is January 31 plus one month?
February has no 31st, so the result is the last day of February: Saturday, February 28, 2026, or February 29 in a leap year such as 2028. The calculator tells you when it has moved a date to the end of a shorter month like this.
How are business days counted?
Business days are Monday to Friday. Adding 10 business days to Monday, October 19, 2026 gives Monday, November 2, 2026, which is 14 calendar days later. Public holidays are not excluded, so add one day for each holiday in the period.
Does the difference between two dates include the end date?
The calendar-day difference is the end date minus the start date, so only one of the two is counted. January 1 to December 31, 2026 is 364 days; add 1 (365) if you count both days, as you would for the length of an event. The day counter has a switch for this.
Why is adding 2 months different from adding 1 month twice?
Because of month ends. January 31, 2026 plus 2 months is March 31. But January 31 plus 1 month is February 28, and February 28 plus 1 month is March 28. Add the whole amount in one step to keep the original day of the month.