Date Calculator: Add or Subtract Days

How to use the date calculator

1. 1

   Set the start date

   It defaults to today. Change it to any past or future date.

2. 2

   Choose add or subtract

   Use “+ Add” to go forward in time and “− Subtract” to go back.

3. 3

   Enter years, months, weeks or days

   Fill in any combination, or tap a quick button like 30 or 90 days. The new date appears instantly with its weekday.

How date math works

Days and weeks are exact: 1 week is always 7 days, so adding them is simple counting. Months and years are calendar units, and they don't all have the same length. Adding one month moves you to the same day number in the next month. When that day doesn't exist, the date is clamped to the last day of the month.

Common spans from January 1, 2027

Months vs. days: quick conversions

Day 0 or day 1? How deadlines are counted

When a letter says “within 14 days of January 10”, the start date is usually day 0. Day 1 is January 11, and day 14 is January 24. That's the method this calculator uses: adding 14 days to January 10 gives January 24.

Some legal and official deadlines have extra rules. A deadline that falls on a weekend or public holiday may move to the next working day, and some courts count “clear days”, which exclude both the first and last day. When it matters, check the exact wording.

Which weekday will it be?

Adding a multiple of 7 days always lands on the same weekday. For any other number, divide by 7 and look at the remainder: that's how many weekdays forward the result moves. Adding 30 days to a Monday gives a Wednesday, because 30 = 4 weeks + 2 days.

This is also why a date moves forward one weekday each year, or two in a leap year: 365 days is 52 weeks plus 1 day.

Counting backwards from a date

Subtracting works the same way in reverse. It's useful whenever you know the end date and need to plan backwards: when to send wedding invitations, when to book a flight, when to start training for a race, or the last day to post a gift so it arrives in time.

Adding days in Excel and Google Sheets

Dates in spreadsheets are numbers, so you can add days directly. With the start date in cell A1:

EDATE follows the same end-of-month rule as this calculator: January 31 plus one month is the last day of February.

Date formats: US, UK and ISO

The same date is written differently depending on where you are, and mixing them up is one of the most common causes of missed deadlines in international emails and spreadsheets. The date picker on this page follows your device's settings, so it shows dates in your local style.

When sharing a calculated date with someone in another country, write the month as a word, or use the ISO format. Both are impossible to misread.

Date math across years

Crossing into a new year works the same as crossing into a new month: the calculator rolls over automatically. Adding 100 days to November 1 lands on February 9 of the next year, and subtracting 60 days from January 15 lands on November 16 of the previous year. Leap years are handled automatically, so a span that includes February 29 gets the extra day.

When you add years to February 29, the result moves to February 28 in years that aren't leap years, because there's no February 29 to land on. Adding 4 years to February 29, 2028 gives February 29, 2032, since 2032 is also a leap year.

Common periods and how they're counted

When you'll need it

Popular date lookups

To count the gap between two dates instead, use the Days Between Dates Calculator.