Date & Time
Add or Subtract Days from Date
Add or subtract days, weeks, months, or years from any date — with correct end-of-month clamping (Jan 31 + 1 month = Feb 28/29).
Month arithmetic clamps to the last valid day (Jan 31 + 1 month = Feb 28/29). Leap years handled automatically.
Adding Months Is Not Adding Days
Day arithmetic is unambiguous; month arithmetic is not, because month lengths differ. "One month after August 31" lands on October 1 in some systems and September 30 in others — there is no August 31 + 1 month in September. Contract and billing dates should state the convention explicitly (end of month, same day-of-month clamped, or fixed 30-day periods). The other recurring trap is timezones on the boundary: adding 24 hours across a DST change yields a 23- or 25-hour shift in local wall time.
How the Dates Are Calculated
- Enter the starting date.
- Enter the amount and unit: days, weeks, months, or years, and the direction (add or subtract).
- Month arithmetic clamps to the last valid day when needed, so no impossible dates like Feb 31.
- The resulting date is shown with its weekday name for planning.
Worked Example
Common Questions
Why does adding one month sometimes shorten the day?
When the source day does not exist in the target month (the 31st in a 30-day month), the date clamps to that month’s last day — the universal convention in legal and accounting contexts.
Does subtracting work the same way?
Yes, with identical clamping rules applied in reverse.
Related Date & Time
Date Difference Calculator
Find the exact duration between two dates in days, weeks, months, and years — with leap years and month lengths handled correctly.
Business Days Calculator
Count working days between two dates, or find the date N business days from today — with configurable weekends and holiday exclusion.
Week Number Calculator
Get the ISO-8601 week number for any date, plus the quarter and day of year — or list every week of a year with start dates.