How to Add or Subtract Days from a Date
Whether you need to calculate a future deadline, plan a project timeline, or find a date weeks from today, adding and subtracting days from a date is a common task. Here is how to do it correctly.
Adding Days to a Date
The simplest case: add a number of days to a given date. If the result crosses into the next month, you need to account for the number of days in each month.
New Date = Start Date + Number of Days
Example: March 15 + 20 days = April 4
March has 31 days, so March 15 + 16 days = March 31. Then 4 more days = April 4.
Adding Weeks, Months, or Years
| Unit | Conversion | Example |
|---|---|---|
| Weeks | Multiply by 7 | 6 weeks = 42 days |
| Months | Varies (28-31 days) | Jan 31 + 1 month = Feb 28 |
| Years | 365 or 366 (leap year) | Feb 29 2024 + 1 year = Feb 28 2025 |
Leap Year Considerations
February has 28 days in a common year and 29 days in a leap year. A leap year occurs every 4 years, except for years divisible by 100 but not by 400. This matters when adding days that cross February.
Example: Adding 60 days to January 15, 2024 (a leap year): January has 16 remaining days, February has 29, March has 15. Result = March 15, 2024.
Subtracting Days from a Date
Subtracting works the same way but backwards. If you need to find what date was 45 days before April 20:
April 20 - 20 days = March 31
March 31 - 25 more days = March 6
Result: March 6
Common Use Cases
- Project deadlines: Adding business days (excluding weekends) to a start date
- Pregnancy due dates: Adding 280 days (40 weeks) to last menstrual period
- Visa expiration: Adding 90 or 180 days to entry date
- Subscription renewal: Adding 30 days to billing date
- Countdown events: Finding the date X days before a wedding, exam, or event
Key Takeaways
- Always account for different month lengths (28, 29, 30, or 31 days)
- Check for leap years when February is involved
- For business days, exclude weekends and holidays
- Use our Date Add/Subtract Calculator for instant, accurate results