Date Add/Subtract Calculator
Add or subtract days from any date. Calculate future or past dates easily with automatic day-of-week detection.
Key Terms
- Date Arithmetic: Mathematical operations on calendar dates
- Day Offset: Number of days to add (positive) or subtract (negative)
- Day of Week: Which day (Monday-Sunday) the result falls on
- Future/Past Calculation: Adding days goes forward, subtracting goes backward
- ISO Date Format: Standard YYYY-MM-DD date representation
What Are These Terms?
Date Arithmetic
Just like you can add or subtract numbers, you can add/subtract days from dates. The calculator handles month boundaries, leap years, and century rules automatically.
Day Offset
The number you enter determines how many days to move from the start date. Positive numbers (e.g., +30) move forward, negative numbers (e.g., -15) move backward.
Day of Week
Knowing which day of the week a future/past date falls on is useful for planning events, meetings, birthdays, or deadlines.
How Does The Tool Work?
Calculation Method
- Start with any date
- Add/subtract specified days
- Calculate resulting date
- Determine day of week
- Display in YYYY-MM-DD format
Real-World Examples
Example 1: Project Deadline
- Start: June 1, 2026
- Days: +45
- Result: July 16, 2026 (Tuesday)
- Use: "Project due 45 days from start"
Example 2: Return Date
- Start: December 20, 2026
- Days: +14
- Result: January 3, 2026 (Friday)
- Use: "2-week vacation return date"
Example 3: Historical Event
- Start: July 4, 2026
- Days: -100
- Result: March 26, 2026 (Tuesday)
- Use: "What was the date 100 days ago?"
Common Use Cases
- Calculate project deadlines
- Plan vacation return dates
- Determine payment due dates
- Schedule recurring events
- Legal notice periods (30/60/90 days)
- Pregnancy due date estimation
- Contract expiration dates