➕ Date Calculator
Add or subtract a duration from any date. Find out what date falls 90 days from now, 3 months before an event, or any other date arithmetic.
Adding and Subtracting Dates
Date arithmetic — adding or subtracting days, weeks, months, or years from a given date — is a surprisingly tricky calculation to do in your head. Adding 90 days to March 15 requires knowing the exact lengths of March, April, and May. Adding 3 months requires knowing whether the target month has as many days as the starting month. This calculator handles all of these edge cases correctly, including leap years and month-end dates.
Why Date Math Is Harder Than It Looks
The Gregorian calendar is irregular by design — months range from 28 to 31 days, leap years occur on a rule with exceptions, and the 7-day week cycle doesn't align neatly with months or years. When you add "one month" to January 31, the result is ambiguous — February doesn't have 31 days, so the answer could be February 28 (or 29), March 2, or March 3 depending on the convention used. This calculator uses the most common standard: rolling back to the last valid day of the target month.
Practical Applications
Date calculators are widely used for legal and contract deadlines (what is 30 days from the date of signing?), medical scheduling (when does a 14-day course of medication end?), project management milestones, subscription and billing cycles, coupon and warranty expiry dates, and calculating when a baby is due based on a last menstrual period. For any deadline-sensitive planning, using a calculator rather than counting manually eliminates errors that could have real consequences.