Date Subtract Calculator

Working backwards from a known date is a common planning task: a lease ending on June 15 requires 30 days notice, so what is the notice deadline? A product launching November 1 needs a 6-week marketing runway, so when should the campaign begin? A 90-day prescription needs a refill before it runs out, so when did it start? This calculator handles all of these by subtracting any combination of days, weeks, and months from a start date to produce the resulting earlier date. Month subtraction handles year boundaries automatically — subtracting three months from February correctly returns November of the previous year — while the UTC-based arithmetic avoids daylight saving time errors.

The start date is first validated with a UTC round-trip check that rejects impossible dates like February 31. Weeks are converted to days (weeks × 7) and added to the day subtraction total. Months are subtracted by passing a negative month offset to JavaScript's Date.UTC() constructor: new Date(Date.UTC(year, month − subtractMonths, day − totalDays)). The Date constructor automatically handles overflow in both directions — subtracting past month 0 rolls back the year, and subtracting past day 1 rolls back the month. The result is formatted as YYYY-MM-DD (ISO 8601) and a human-readable total day count is calculated for reference.