Add or Subtract from a Date

How to add or subtract time from a date

Start with a date, choose whether you want to go forward (add) or backward (subtract), then type an amount and pick a unit — days, weeks, months, or years. The resulting date pops up immediately, along with the day of the week it lands on and how far it is from today. It is the quickest way to answer questions like “what date is 45 days from now?” without counting on your fingers or a wall calendar.

Days and weeks are the simple cases

Adding days just moves the date forward by that many calendar days, rolling over month and year boundaries for you. Weeks work the same way — they are simply multiples of seven days — so “3 weeks” is always exactly 21 days. There are no surprises here: every day is the same length as far as the calendar is concerned.

What happens at the end of a month

Months and years are where date math gets interesting. When you add a month and the target month is shorter than your starting day, the result is clamped to the last valid day of that month. So January 31 plus one month becomes February 28 (or February 29 in a leap year), because there simply is no February 31. This “clamp to month-end” behaviour is the same convention spreadsheets and calendar apps use, so the answers line up with tools you already trust.

One thing worth knowing: because of clamping, adding a month and then subtracting a month will not always bring you back to the exact day you started on if that day was near the end of a month. That is expected, not a bug — it is just what happens when months have different lengths.

Adding years and the February 29 case

Adding a year keeps the same month and day where possible. The only special case is February 29: if you add a year (or any whole number of years) and land on a non-leap year, the date clamps to February 28, since that year has no 29th. Leap years follow the standard rule — divisible by 4, but not by 100 unless also divisible by 400 — so the tool always knows which Februaries have that extra day.

Counting backward in time

Subtracting works exactly like adding, just in the other direction. Choose “Subtract” to find a date in the past — handy for working out a start date that is a set number of weeks before a deadline, or checking when a warranty period began. The same month-end and leap-year handling applies whichever way you go.

Calendar days, not business days

This tool counts every day, including weekends and public holidays. If you need to skip Saturdays and Sundays — for shipping estimates, court deadlines, or office turnaround times — use the Business Days calculator instead, which moves forward only on working days.