Date Add or Subtract Calculator

This calculator moves a date forward or backward by a number of days, weeks, months or years and shows the resulting date, its weekday and how many calendar days separate the two. It handles month lengths and leap years for you.

Common uses are notice periods ("30 days' notice"), return and warranty windows, visa and probation periods, medication courses, and counting back from an event, such as working out the date six weeks before a wedding or exam. For pregnancy dates use the due date calculator, which follows obstetric conventions.

Days and weeks are unambiguous. Months and years are not, because months differ in length: this calculator rolls an impossible date into the next month, so January 31 plus one month gives March 3 in 2026 (March 2 in a leap year). Spreadsheets and many contracts instead stop at the last day of the shorter month (February 28). If the exact convention matters, read the section below and check the wording of your agreement. Weekends are counted like any other day here; to count working days only, use the business days calculator.

Adding months keeps the day-of-month and changes the month. When the target month is too short, there are two common conventions:

• Roll over (this calculator): the extra days spill into the next month. January 31 + 1 month = March 3 (2026). February 29, 2028 + 1 year = March 1, 2029.
• Clamp to month end (Excel's EDATE, many banks and contracts): the date stops at the last day of the shorter month. January 31 + 1 month = February 28; February 29, 2028 + 1 year = February 28, 2029.

The two only differ when the start date is the 29th, 30th or 31st. For anything legal or financial on those dates, follow the convention in the document, or count in days instead.

Use this to check a result by hand or to see how many days "one month" really is from a given start date. A year is a leap year if it divides by 4, except century years, which must divide by 400: 2028 is a leap year, 2100 will not be.

These are often used as if they were the same, but they are not. Four weeks is always 28 days. One calendar month is 28 to 31 days depending on the start date. "30 days" is a fixed count that ends on a different date of the month each time. A 12-month period is 365 or 366 days, while 52 weeks is 364 days, which is why weekly schedules drift by a day or two each year. To measure the exact gap between two dates you already know, the date difference calculator gives days, weeks and months.