Date Calculator
Add or subtract days, weeks, months, or years from a date, or calculate the difference between two dates in various time units.
About this Calculator
Add or subtract days, weeks, months, or years from a date, or calculate the difference between two dates in various time units.
Formula & Calculations
Formula
Result date = Start date ± offset; Difference = End date − Start dateWhere:
- Start Date=The reference date for calculation
- End Date=The second date for difference calculation
- Offset=Number of days, weeks, months, or years to add or subtract
- Difference=Time span between two dates expressed in days, weeks, months, and years
Assumptions
- Uses the Gregorian calendar system.
- Month arithmetic handles varying month lengths correctly (e.g., adding 1 month to Jan 31 yields Feb 28/29).
- Leap years are accounted for automatically.
- The difference mode calculates total days first, then breaks down into weeks, approximate months, and years.
Calculation Examples
Example 1
Adding 3 months to January 15 yields April 15.
Example 2
Adding 1 month to January 31 lands on February 29 in a leap year, since February has only 29 days.
Example 3
The total number of calendar days between the two dates is 152.
Frequently Asked Questions
Does this calculator account for leap years?
Yes. The calculations use JavaScript's native Date object which correctly handles leap years, including the February 29 addition every 4 years.
How does adding months handle different month lengths?
When adding months, the calculator uses JavaScript's setMonth() method, which automatically caps the day to the last valid day of the target month. For example, Jan 31 + 1 month = Feb 28 (or 29 in leap years).
Can I calculate business days?
This calculator works with calendar days. For business-day calculations that exclude weekends and holidays, a specialized business-day calculator would be needed.