Date Calculator
Pick a start date, choose Add or Subtract, and enter any mix of years, months, weeks and days. The new date appears at once, with its weekday, its day of the year and its ISO week. It handles month ends and leap years the way a spreadsheet does. Everything runs in your browser.
The start day does not exist in the new month, so it moved back to the last day of that month.
Years and months move first, as whole months. When the new month is shorter than the start day, the date moves back to its last day, so 31 January plus one month is 28 February, or 29 in a leap year, the way the spreadsheet function EDATE works. For that reason adding and then subtracting the same months does not always return the start date. Weeks and days are then counted as plain days. Dates are whole calendar days with no time of day or time zone, so a daylight saving change cannot move the result. The calendar is the Gregorian calendar, extended back before 1582, and years 1 to 9999 are supported. Nothing you enter is uploaded.
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Developer & WebAbout the Date Calculator
Years and months are added first, as whole months. When the new month is shorter than the start day, the date moves back to the last day of that month, so 31 January plus one month is 28 February, or 29 February in a leap year, and 29 February 2024 plus one year is 28 February 2025. This is how the spreadsheet function EDATE works, and the tool tells you when it moved a date back. Weeks and days are then counted as plain days: a week is 7 days. Because month lengths differ, adding and then subtracting the same months does not always return the start date, for example 31 January plus one month minus one month is 28 January. Subtracting works the same way in reverse, so 31 March minus one month is 28 February in 2026. The calendar is the Gregorian calendar extended back before 1582, years 1 to 9999 are supported, and dates are whole days with no time of day and no time zone, so a daylight saving change can never move a result. For the number of days between two dates, use the Days Between Dates Calculator, and for working days only, the Business Days Calculator. Nothing you enter is uploaded.
How do I add days to a date?▾
Enter the start date, leave Add selected and type the number of days. For 100 days after 25 October 2026 the tool gives 2 February 2027. You can mix amounts, such as 1 year, 2 months and 10 days.
What is 31 January plus one month?▾
The last day of February: 28 February, or 29 February in a leap year. A month cannot have a day that does not exist in it, so the date moves back to the month's last day, as the spreadsheet function EDATE does.
How does it handle leap years?▾
Adding a year to 29 February gives 28 February of the next year unless that year is also a leap year, so 29 February 2024 plus 1 year is 28 February 2025, and plus 4 years is 29 February 2028.
Why does adding and then subtracting a month not always return the same date?▾
Because months have different lengths. 31 January plus one month is 28 February, and 28 February minus one month is 28 January, not 31 January. Day amounts are exact and always reverse, but month amounts can move a day.
How do I count the days between two dates instead?▾
Use the Days Between Dates Calculator, which counts the days from one date to another, or the Business Days Calculator to count working days only. This tool works from one date and an amount.