Add or Subtract Days
Calculate a future or past date from any starting date
Frequently Asked Questions
Select your start date, choose "Add (+)" as the operation, enter the number of days (and optionally weeks, months, or years), then click Calculate. The tool shows the resulting future date along with the day of the week.
Yes. Adding 45 days to December 15 will correctly roll over into the next year. The calculator handles varying month lengths (28–31 days), leap years, and year boundaries automatically.
Yes. Switch the operation to "Subtract (−)" and enter the number of days, weeks, months, or years. The calculator will compute the date that many days in the past from your start date.
Add or Subtract Days Calculator: The Complete Guide to Date Arithmetic
Date arithmetic—adding or subtracting days, weeks, months, or years from a date—is something we all need but rarely find easy to do mentally. "What date is 90 days from today?" "When is 6 months before my lease expires?" "What was the date 45 days ago?" Our Add/Subtract Days Calculator handles all of these scenarios instantly, accounting for varying month lengths, leap years, and year boundaries.
How Date Arithmetic Works
Adding Days: Result Date = Start Date + N days. The calculator advances day-by-day through the calendar, rolling over month and year boundaries as needed. Adding 31 days to January 15 crosses into February (and accounts for whether it's a leap year).
Adding Months: Result Date = Same day in a future month. January 31 + 1 month = February 28 (or 29 in leap years), because February doesn't have 31 days. This "end-of-month" adjustment is standard in date arithmetic.
Adding Months: Result Date = Same day in a future month. January 31 + 1 month = February 28 (or 29 in leap years), because February doesn't have 31 days. This "end-of-month" adjustment is standard in date arithmetic.
Worked Examples
Example 1 — Project Deadline: Your project starts on March 10, 2026, and you have a 90-day deadline. Adding 90 days: March has 21 remaining days, April has 30, May has 31, leaving 8 days into June. Result: June 8, 2026 (Monday). You now know your exact deadline, including the day of the week for scheduling the final review.
Example 2 — Return/Refund Window: You purchased an item on November 15 with a 30-day return policy. Adding 30 days: November has 15 remaining days, leaving 15 days into December. Result: December 15. If you're returning during the holiday rush, knowing the exact last day avoids disputes.
Example 3 — Warranty Expiration: Your appliance warranty is 2 years from purchase date of July 22, 2024. Adding 2 years: July 22, 2026 (Wednesday). If a repair is needed, you know exactly whether you're still covered.
Common Date Additions: Quick Reference
| Duration | Days | Common Use |
|---|---|---|
| 1 week | 7 | Weekly deadlines, follow-up appointments |
| 2 weeks | 14 | Pay periods, two-week notice |
| 30 days | 30 | Return policies, billing cycles, trial periods |
| 45 days | 45 | Mortgage contingencies, escrow timelines |
| 60 days | 60 | Insurance enrollment windows, COBRA deadlines |
| 90 days | 90 | Probation periods, quarterly reviews, visa durations |
| 120 days | 120 | Construction timelines, project milestones |
| 180 days | 180 | Half-year reviews, statute of limitations |
| 365 days | 365 | Annual renewals, warranties, subscriptions |
| 2 years | 730–731 | Extended warranties, lease terms |
| 5 years | 1,826–1,827 | Vesting schedules, statute of limitations |
Month-Boundary Handling
Adding months is trickier than adding days because months have different lengths. Here's how edge cases are typically handled:
- January 31 + 1 month: February 28 (or 29 in leap years). Since February doesn't have a 31st, the result "clamps" to the last day of February.
- March 31 + 1 month: April 30. Same principle—April has only 30 days.
- January 29 + 1 month: February 28 in non-leap years, February 29 in leap years.
- August 31 − 1 month: July 31. This one works cleanly because July has 31 days.
Legal "Month" Definition: In legal contexts, "one month from January 31" can mean different things depending on the jurisdiction. Some courts use the "corresponding date" rule (February 28/29), while others count 30 days. If you're calculating legal deadlines, verify which convention your jurisdiction uses.
Leap Year Considerations
Leap years affect date arithmetic in specific scenarios:
- Adding 365 days from January 1 of a leap year lands on December 31 (not January 1 of the next year), because the leap year has 366 days.
- Adding 1 year from February 29 lands on February 28 of the next non-leap year, since February 29 doesn't exist in non-leap years.
- Subtracting 1 year from March 1 of a leap year gives March 1 of the previous year—it doesn't "jump" to February 29.
Calendar Days vs. Business Days: This calculator counts all calendar days, including weekends and holidays. If you need to calculate business days only (excluding weekends and optionally holidays), use our Business Days Calculator.
Our Add/Subtract Days Calculator is free and runs entirely in your browser. For related tools, try Date Difference Calculator, Days Between Dates, and Business Days Calculator.