Date ± Days Calculator

What the Add or Subtract Days Calculator Does

This calculator takes a starting date and a number of days, then returns the resulting calendar date and its weekday. Choose to add days (move forward in time) or subtract days (move backward), and the tool counts every calendar day in between, including weekends and holidays.

It is useful for anyone working with deadlines or waiting periods: project managers tracking a 90-day milestone, freelancers calculating a net-30 invoice due date, students counting days until an exam, or anyone tracking a shipping window, warranty period, or return cutoff.

How It Works: The Formula

The math is simple addition or subtraction on a date value. Written out:

The key detail is that the calculator uses calendar days, not business days. It automatically handles month lengths (28, 29, 30, or 31 days), rolls over into the next or previous month and year as needed, and accounts for leap years (February 29 in years divisible by 4, except centuries not divisible by 400).

Once the result date is found, the weekday is determined by counting the day-of-week offset from the start date, since exactly 7 days returns the same weekday.

• Add: Result Date = Start Date + Number of Days
• Subtract: Result Date = Start Date - Number of Days

Worked Example

Suppose you issue an invoice on March 15, 2026 with net-45 payment terms, and you want the due date.

Start with March 15. March has 31 days, so 31 - 15 = 16 days remain in March. Subtract those from 45 to leave 29 days. April has 30 days, so after stepping through all of April you have used 16 + 30 = 46 days, which overshoots. Instead, after consuming the 16 remaining March days you land on March 31, then add the leftover 29 days into April: March 31 + 29 days = April 29, 2026.

The calculator returns April 29, 2026, which falls on a Wednesday. Doing this by hand is error-prone, especially across month and year boundaries, which is exactly what the tool removes.

Calendar Days vs. Business Days

The most common mistake is assuming the result skips weekends or holidays. This calculator counts calendar days, so a 10-day result spans two weekends and includes them in the total.

If your contract or deadline is measured in business days (working days), this tool is not the right fit on its own, because business-day counts exclude Saturdays, Sundays, and public holidays. Always check the wording of your agreement before relying on a date.

Tips and Factors That Affect the Result

A few details change the outcome more than people expect. Keep these in mind when you enter your numbers:

• Inclusive vs. exclusive counting: decide whether the start date counts as day 0 or day 1. Net-30 usually means 30 days after the invoice date (start date excluded).
• Leap years: adding 365 days from a date before February 29 in a leap year lands one day earlier than you might expect.
• Direction matters: subtracting days moves into the previous month or year, where shorter months can shift the result.
• Time zones: this tool works on calendar dates only, so it does not account for time-of-day or time-zone shifts.
• Double-check edge dates like the 31st or end of February, where month-length rollover most often causes hand-calculation errors.