Date Calculator

How dates are added and subtracted

Days and weeks are exact: a day moves the date by one, a week by seven. Months and years are not a fixed number of days, so they are counted on the calendar instead. Adding one month moves to the same day of the next month, and one year to the same date next year, whatever the number of days in between.

When you enter several units, the calculator adds years and months first, then weeks and days. The order matters near month ends: 30 January + 1 month + 2 days is 2 March (28 February, then two days), while adding the days first would give 1 March.

With business days only ticked, the calculator counts Monday to Friday and steps over Saturdays and Sundays. The start date itself isn’t counted, so one business day after a Friday is the following Monday. Public holidays vary by country, region and employer and are not skipped.

The rules the calculator follows

If the day doesn’t exist in the target month, the result is that month’s last day. This is how monthly due dates usually work:

Days are added by converting the date to a serial day number, adding, and converting back (Howard Hinnant’s civil-date algorithm), so leap years — every year divisible by 4, except century years not divisible by 400 — are always right. For business days, each full block of 5 business days covers exactly 7 calendar days, and the last few days are stepped one at a time.

Worked examples

1. A month from 31 January 2026. February 2026 has 28 days, so there is no 31 February and the result is the month’s last day: Saturday, 28 February 2026. In 2024, a leap year, the same sum gives 29 February.
2. 90 days from 15 September 2026. The rest of September is 15 days, October adds 31 and November 30, making 76; 14 more days reach Monday, 14 December 2026.
3. 100 days before 15 September 2026 is 7 June 2026.
4. 10 business days after Tuesday, 15 September 2026. Two full working weeks pass over two weekends (4 days), so the answer is 14 calendar days later: Tuesday, 29 September 2026.

Things to watch for

• Months don’t reverse cleanly. 31 March − 1 month = 28 February, but 28 February + 1 month = 28 March. Adding and subtracting days always round-trips; months at the end of a month do not.
• Four weeks is not a month. Four weeks is always 28 days; a month is 28 to 31 days.
• Deadlines may count differently. This calculator doesn’t count the start date. Some contracts, courts and agencies count “within 30 days” differently or move a deadline that lands on a weekend or holiday. Check the wording that applies to you.
• Holidays. For a working-day count that excludes public holidays, add one business day for each holiday in the period.