Add or Subtract Days to a Date Calculator

Counting forward or backward on the calendar

This answers questions like “what date is 90 days from today,” “when is 6 weeks before my due date,” or “what was the date 18 months ago.” It takes a starting date and shifts it by a number of days, weeks, months, or years.

Days and weeks are the easy part

Adding days or weeks is pure counting: weeks are just days times seven. The calendar’s irregular month lengths and leap years are handled automatically because the calculation works on the actual date object, not a fixed 30-day month.

Months and years need a rule for the end of the month

Adding months is where it gets subtle. What is one month after January 31? February has no 31st. The standard behavior, which this calculator follows, is to land on the last valid day, so January 31 plus one month gives the end of February. Adding a year to February 29 (a leap day) lands on February 28 in a normal year. These are the cases that trip up people doing it by hand.

What you get back

The resulting calendar date, the day of the week it falls on, and the total number of actual days the shift moved you. For months and years the day total varies (a year is 365 or 366 days depending on whether a leap day sits in the span), so the tool reports the real count rather than an estimate.

Worked example

Start on March 15, 2024 and add 90 days. The result is June 13, 2024, a Thursday. Because 2024 is a leap year the span still counts a true 90 calendar days.

Common uses

Deadlines and notice periods, contract and lease end dates, return-by and warranty dates, pregnancy and project milestones, and “X days from now” reminders.