Add or Subtract Days from a Date

Need to know what date falls 30 days from now, 6 weeks before a deadline, or 3 months after a bill? This calculator takes any date and shifts it forward or backward by days, weeks, months or years — the inverse problem to "how many days between two dates," where instead of counting a gap you project onto the calendar.

How it works

There are two very different kinds of arithmetic at play, and knowing the difference prevents many common errors:

• Days and weeks — pure day arithmetic. Adding or subtracting 30 days means stepping 30 calendar days, no matter how the month lengths fall. Weeks are simply days × 7. The result lands on the exact calendar date, and the calculator handles leap years and month boundaries automatically along the way.
• Months and years — calendar-based, not day-based. Adding 1 month to 31 January does not mean "30 days later"; it means moving to the same day in the next month. When that target month has fewer days than your start day, the result rolls over to the last valid day (28 or 29 February, or the 30th for a 30-day month). This is the standard convention used by spreadsheets, contracts and accounting systems.

Worked examples

• 14 August 2026 + 30 days = 13 September 2026 (not 14 September, because August has 31 days)
• 14 August 2026 − 6 weeks = 3 July 2026 (42 days back)
• 31 January 2026 + 1 month = 28 February 2026 (January has 31 days, February has 28 in 2026)
• 31 January 2024 + 1 month = 29 February 2024 (2024 is a leap year, so February has 29)
• 14 August 2026 + 5 years = 14 August 2031

Common uses

• Contract and notice periods — "45 days from signing" and "3 months after completion" are everywhere in leases, employment and service agreements.
• Medicine refills and prescriptions — knowing exactly when a 30-day supply runs out.
• Billing, EMI and subscription renewals — monthly commitments synced to a fixed day of the month.
• Warranty and visa validity windows — where a few days of miscalculation can mean a lapsed warranty or an overstayed visa.
• Countdown and event planning — working backward from a fixed event date to set reminders, delivery cut-offs and prep milestones.

Pitfalls to watch for

• Month rollover surprises — adding one month repeatedly from the 29th, 30th or 31st can "drift" or clamp. For example, starting 31 January, a naive hands-on calculation can produce duplicate months. Tools like this use the standard clamp-to-last-day rule, but if a contract dates from a clamped result, double-check the intent.
• Leap-year differences — the same date operation can give different results in leap versus common years, which matters when projecting across February.
• Day vs. month semantics — always confirm whether "a month" in your context means 30 fixed days or one calendar month; the two can differ by a day or more.

If you need the number of days between two known dates instead, use the Days Between Dates calculator.

Historical note

Because "a month" has never meant a fixed number of days, calendar arithmetic has always required convention rather than pure arithmetic — a reason why early legal codes specified notice periods in both months and days, nudging toward clearer, fixed-day counts.