Date Calculator
Add or subtract days, weeks, months, or years from any start date. Calculate future or past target dates with business day exclusion.
Projected Target Date
Day of the week will appear here.
Friday, September 18, 2026
How to Calculate Future or Past Dates (Step-by-Step Guide)
To calculate a target calendar date by adding or subtracting calendar days and weeks, use this date calculation method:
- Step 1: Convert weeks into equivalent days by multiplying the weeks input by 7 (e.g. 3 weeks = 21 days).
- Step 2: Sum all offset days (e.g. 90 days + 21 days = 111 total offset days).
- Step 3: Step forward (or backward) by the total number of days, taking into account leap years (29 days in February) and months of varying lengths (28, 30, or 31 days).
Formal Date Offset Equations
Total Offset Days (D) = Days + (Weeks × 7)Target Date (Add) = Start Date + D Days (accounting for Gregorian calendar months)Target Date (Subtract) = Start Date − D Days (accounting for Gregorian calendar months)
Common Date Addition & Subtraction Reference Table
Standard pre-calculated date offset reference values showing target results from sample anchor dates.
| Anchor Start Date | Offset Action | Offset Amount | Resulting Target Date |
|---|---|---|---|
| January 1, 2026 | Add (+) | 30 Days (4w 2d) | January 31, 2026 |
| January 1, 2026 | Add (+) | 90 Days (12w 6d) | April 1, 2026 |
| January 1, 2026 | Add (+) | 180 Days (25w 5d) | June 30, 2026 |
| July 4, 2026 | Subtract (−) | 30 Days (4w 2d) | June 4, 2026 |
| July 4, 2026 | Add (+) | 100 Days (14w 2d) | October 12, 2026 |
| December 25, 2026 | Subtract (−) | 90 Days (12w 6d) | September 26, 2026 |