Pick a base date, type an amount (negative to go back in time), choose a unit, and the result date updates instantly along with the weekday it lands on. Days and weeks are exact arithmetic. Months and years move the calendar field directly, which raises the classic month-end question: January 31 plus one month has no February 31, so this tool clamps the overflow back to the last valid day of the target month — Jan 31 + 1 month becomes Feb 28 (or Feb 29 in a leap year). The same clamping applies to leap-day anniversaries.
All math uses your local calendar with no time component, so time zones and daylight-saving changes never shift the result by a day. This is useful for due dates, contract terms, subscription renewals and reminders. Everything is computed in your browser and nothing is sent anywhere.