Free Online Date Add/Subtract Calculator
Add or subtract days, weeks, months, or years from a date.
When a month does not have the target day, the result is clamped to the last valid day of that month. For example, adding one month to January 31 gives February 28 in a normal year and February 29 in a leap year. Days are then applied to the clamped date.
About the Date Add/Subtract Calculator
Adding or subtracting time from a date sounds simple until month lengths and leap years get involved. If you add one month to January 31, there is no February 31. If you add one year to February 29, the next year may not have that day at all. This calculator handles those cases with month-end clamping and returns a precise result date, the weekday it falls on, an ISO-formatted value, and the number of days between the original date and the result.
How the calculation works
Enter a start date, choose whether to add or subtract, then provide any combination of years, months, weeks, and days. Years and months are combined into a single month offset so the order is predictable. The calculator moves the calendar month first and keeps the original day number when that day exists in the target month. If the target month is shorter, the day is clamped to the last day of that month. Weeks and days are then applied as exact day offsets. Choosing subtract reverses the whole operation, so the result is symmetric.
Month-end clamping and leap years
Clamping is the rule that keeps a date valid. Moving one month forward from January 30 or January 31 lands on the last day of February, which is the 28th in a common year and the 29th in a leap year. The same rule applies when subtracting months from a 31-day date into a 30-day month. The tool reports the operation it performed so you can see whether clamping occurred. Because the result is a real calendar date, it is safe to use for contracts, deadlines, billing cycles, and appointment scheduling, although a legal agreement may define its own counting convention.
Days difference
The days difference is the signed distance between the start date and the result, measured in whole calendar days. It is not a count of business days and it does not exclude weekends or holidays. For a working-day calculation, use a dedicated business-day tool. Keeping the difference visible is helpful when you want to confirm the combined effect of several offsets, such as moving forward 18 months and back 10 days at the same time.
Everyday uses
People reach for date arithmetic for renewals, notice periods, warranty expiry, visa stays, prescription refills, and project milestones. A landlord may add a year to a tenancy start, a supplier may subtract a fortnight from a delivery deadline, and an administrator may count forward ninety days from an incident. Whenever a policy specifies a calendar interval, confirm whether the anniversary itself is inclusive or exclusive, because that single day can alter a statutory deadline. Timezone shifts and daylight saving do not change a pure calendar date, but they matter once a precise hour is attached.
Frequently asked questions
What happens when I add a month to January 31?
The result is clamped to the last day of February, which is February 28 in a common year or February 29 in a leap year.
Are years and months applied before days?
Yes. Years and months are combined into one month offset and applied first, then weeks and days are applied to the clamped result.
Does the calculator account for daylight saving time?
It works with calendar dates rather than timestamps, so daylight saving changes do not affect the result.