Date Add & Subtract Calculator

To shift a calendar date forward or backward, add the interval to the day, week, or month count. Weeks are exact multiples of seven days. Months follow calendar months — adding one month to January 31 gives February 28 or 29, not March 3.

This tool uses calendar arithmetic, not business days. Weekends and holidays are included unless you use the business-days calculator. For legal deadlines that say "30 calendar days," count from the day after the event unless your policy says otherwise.

Example: starting March 15, 2026 and adding 45 days lands on April 29, 2026. Subtracting 3 months from July 31, 2026 gives April 30, 2026 — month-end dates adjust to the last valid day of the target month.

Month arithmetic clamps to the end of the target month, which is why January 31 plus one month returns February 28 rather than spilling into March. Every major date library behaves this way, but it means the operation is not reversible: February 28 minus one month gives January 28, not the January 31 you started from.

Order of operations changes the answer when months and days are mixed. January 30 plus one month plus two days gives March 2, because the month step clamps to February 28 first. Reverse the steps — plus two days, then plus one month — and you land on March 1. Decide which unit applies first before quoting a deadline.

Court and contract deadlines usually exclude the triggering day and then extend past closures. US Federal Rule of Civil Procedure 6(a) counts from the day after the event and, when the last day falls on a weekend or legal holiday, runs the period to the next business day — so a calendar-day result may need one more step.

Subtracting walks backward under the same month-length rules, so a "three months before" milestone taken from a month-end date lands on the last day of the earlier month. For notice periods stated in months, check whether your contract means calendar months or a fixed 30 or 90 days, because the two diverge by up to three days.

Weeks are the one unambiguous unit here: they are always exactly seven days, so eight weeks is 56 days in every month and every year, which is why clinical and payroll schedules prefer weeks over months.

Leap years add February 29 — adding days across February in a leap year shifts results compared with non-leap years. The calculator handles calendar rules automatically once dates are entered correctly.