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How to Add or Subtract Days, Weeks, and Months from a Date
Formula
Adding or subtracting time from a date is one of the most common date arithmetic tasks. Whether you need to find a deadline 90 days from now, determine a date 6 weeks in the past, or calculate when a 3-month subscription expires, this calculator handles all of it. Enter your starting day-of-year and specify the days, weeks, and months to add or subtract, and the tool computes the resulting day-of-year instantly.
The calculation converts weeks to days (multiplying by 7) and months to an approximate 30-day equivalent, then sums everything together. While real calendar months vary between 28 and 31 days, the 30-day approximation is standard for quick planning and estimation. For precise monthly calculations involving specific calendar months, consult a full calendar tool. This calculator is ideal for fast, approximate date arithmetic.
Common use cases:
- Calculating deadlines and due dates for projects
- Determining expiration dates for warranties or subscriptions
- Planning future events a certain number of weeks or months away
Frequently Asked Questions
Dr. David Park
Applied Mathematician, PhD Mathematics
David holds a PhD in Applied Mathematics from MIT. He has published research on numerical methods and computational algorithms used in engineering and scientific calculators.
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