Date calculator: add or subtract from a date

How the date calculator counts

Adding days is simple arithmetic: every day is the same length. Months and years are not, so the calculator works in a fixed order. It adds the years and months first, then the weeks and days. That's the order people, contracts and most software use, and it matters at the end of a month. Take January 30, 2026 plus 1 month and 2 days:

• Months first (what the calculator does): January 30, 2026 + 1 month = February 28, 2026, then + 2 days = March 2, 2026.
• Days first: January 30, 2026 + 2 days = February 1, 2026, then + 1 month = March 1, 2026.

Weeks are always 7 days, so 6 weeks and 42 days give the same answer. The start date counts as day zero: adding 1 day to a Monday gives Tuesday.

Month ends and leap years

When you add months, the day of the month stays the same if it can. If the new month is too short, the date moves back to its last day. The calculator shows a note whenever this happens, so you know the answer is the last day of the month rather than a mistake. Adding 1 month to the last days of January:

Subtracting works the same way: March 31, 2026 minus 1 month is February 28, 2026. Leap days follow the same rule. A leap year has 366 days, with February 29, and comes every 4 years, except century years that can't be divided by 400 (2000 was a leap year, 2100 won't be). So February 29, 2028 plus 1 year is February 28, 2029, while plus 4 years is February 29, 2032.

Calendar days vs business days

Turn on business days when a deadline is given in working days, like "within 10 business days". The days part then skips Saturdays, Sundays and public holidays. From Friday, November 20, 2026, 10 calendar days is November 30, 2026, but 10 business days is December 7, 2026, because the count skips the weekends and Thanksgiving. With the England and Wales calendar, 10 business days from December 18, 2026 is January 6, 2027: Christmas Day, the Boxing Day substitute day on December 28 and New Year's Day are all skipped. The holidays come from the official rules: US federal holidays under 5 U.S.C. 6103 with their observed days, and UK bank holidays from GOV.UK.

What people use it for

• Contract terms and renewals: a 12-month agreement starting January 31, 2026 reaches its anniversary on January 31, 2027. Many contracts end the day before, so check the wording.
• Notice periods and probation: 2 weeks' notice, a 90-day review or a 6-month probation period. Remember that 90 days and 3 months aren't the same.
• Project schedules: add 6 weeks for a phase, then business days for the sign-off.
• Looking back: subtract to find when a 30-day return window or a 1-year warranty started.

For a quick "what's the date 30 days from now", the days from today calculator is one step shorter. To count the days between two dates you already have, use the days between dates calculator. Working with clock times instead? Try the hours calculator or the time calculator.