Date Calculator
Add or subtract days, weeks, months, and years from any date or calculate the exact duration between two dates.
ISO Format: 2026-10-05
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What Is a Date Calculator?
A date calculator performs calendar arithmetic, allowing users to project forward or backward in time by adding or subtracting specific numbers of days, weeks, months, or years from an initial starting date. It accounts for variable month lengths (28, 29, 30, or 31 days) and Gregorian leap years.
How to Use This Calculator
- Select your starting calendar date.
- Choose whether to add (+) or subtract (-) time.
- Enter the number of years, months, weeks, or days to adjust.
- Instantly view the resulting calendar date, day of the week, and ISO format.
Calendar Projection Logic
D_{\text{target}} = D_{\text{start}} \pm (365.25 \cdot Y + 30.44 \cdot M + 7 \cdot W + D)Calendar arithmetic applies variable month end clipping and Gregorian 400-year leap rules to ensure accurate date projections.
Worked Example
Scenario: A contract starts on March 15, 2025 and runs for 90 days.
Start date: March 15, 2025.
Add 90 calendar days.
March has 16 remaining days, April has 30 days, May has 31 days (total 77 days).
90 - 77 = 13 days into June.
Tips & Key Notes
- When adding months to dates ending on the 31st (e.g., January 31 + 1 month), dates automatically clip to the last day of February (Feb 28 or 29).
- Business contracts often require adding business days rather than calendar days; check our business days counter if excluding weekends.
- Leap day (February 29) occurs only on years divisible by 4, except century years not divisible by 400.
Frequently Asked Questions
How do you add 30 days to a date?
To add 30 days to a date, increment the day counter by 30 while advancing the month whenever the current month’s total days (28 to 31) are exceeded.
Does this date calculator support leap years?
Yes, our engine automatically accounts for February having 29 days during leap years (such as 2024, 2028, and 2032).
Can I calculate the number of days between two dates?
Yes, switch to the "Days Between Dates" mode to see total calendar days, business days, and weekend days between any two dates.
Why does adding one month not always equal 30 days?
Months in the Gregorian calendar vary between 28 and 31 days. Adding 1 month moves to the equivalent day number in the following month regardless of day count.
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