What This Calculator Does

This date calculator has two modes. The Date Difference mode calculates the exact duration between two dates in years, months, days, total days, and weeks. The Add/Subtract mode lets you add or subtract a specified number of days, weeks, months, or years from a starting date to find the resulting date.

Inputs Explained

Start Date: The beginning date for your calculation, in YYYY-MM-DD format.
End Date (Difference mode): The ending date. The calculator finds the duration between the start and end dates.
Amount (Add/Subtract mode): A whole number of units to add or subtract from the start date.
Unit: The time unit for addition or subtraction — Days, Weeks, Months, or Years.
Operation: Choose Add (+) to move forward in time, or Subtract (−) to move backward.

How It Works

For date differences, the calculator uses the Luxon library to compute the interval between two dates. It first extracts full years, then full months from the remainder, then the leftover days. Total days and weeks are calculated as simple divisions. For the Add/Subtract mode, the specified number of units is added to (or subtracted from) the start date using calendar-aware arithmetic that handles month-end boundaries and leap years.

Formulas Used

Date Difference = End Date − Start Date → Y years, M months, D days
Total Days = floor(End Date − Start Date) in days
Total Weeks = floor(Total Days ÷ 7)

Add/Subtract: Result Date = Start Date ± Amount × Unit

Date Calculator

Calculate duration between two dates.

Add or subtract time from a date.

Start Date

Unit

Step-by-Step Example: Date Difference

Start Date: March 15, 2024

End Date: January 31, 2026

Step 1: Full years from Mar 15, 2024 to Mar 15, 2025 = 1 year.

Step 2: Full months from Mar 15, 2025 to Jan 15, 2026 = 10 months.

Step 3: Remaining days from Jan 15 to Jan 31, 2026 = 16 days.

Result: 1 year, 10 months, 16 days. Total: 687 days (98 weeks, 1 day).

Step-by-Step Example: Add Days

Start Date: January 31, 2026

Operation: Add 90 Days

Result: May 1, 2026 (Thursday)

Use Cases

Project management: Calculate deadlines by adding working days or weeks to a project start date.
Pregnancy tracking: Find your estimated due date by adding 280 days (40 weeks) to the first day of your last period.
Legal deadlines: Compute filing deadlines, notice periods, or statute of limitations expiration dates.
Event planning: Count down the days until a wedding, graduation, or holiday.
Financial calculations: Determine the exact number of days between two dates for interest calculations.

Assumptions and Limitations

All calculations use the Gregorian calendar. Julian calendar dates are not supported.
When adding months, if the resulting day exceeds the target month's length (e.g., Jan 31 + 1 month), the date rolls to the last valid day of the target month.
Week calculations are based on 7-day blocks and do not account for business days or weekends.
The calculator does not account for holidays or business-day conventions.
Leap years are handled correctly — February 29 is included when applicable.

Frequently Asked Questions

How are days between two dates calculated?

The calculator computes the full calendar days from the start date to the end date. It uses the Luxon date library for calendar-aware arithmetic that accounts for different month lengths and leap years. The result is expressed both as a years-months-days breakdown and as a total day count.

Does the date calculator count the start date or end date?

The calculator counts the gap between the two dates. For example, from January 1 to January 2 is 1 day. Neither the start date nor the end date is double-counted. This is consistent with how most date difference tools and spreadsheet functions like Excel's DATEDIF work.

What happens when I add one month to January 31?

Adding one month to January 31 gives February 28 (or February 29 in a leap year), because February does not have a 31st day. The calculator uses the Luxon library's month-end clamping behavior, which is the standard approach used by most calendar software.

Can I calculate business days between two dates?

This calculator counts calendar days, not business days. It does not exclude weekends or public holidays. For business day calculations, you would need to subtract weekends and applicable holidays from the total day count provided.

How do I find a date that is a specific number of weeks in the future?

Use the Add/Subtract tab. Enter your starting date, select 'Add (+)', type the number of weeks, and choose 'Weeks' as the unit. The calculator will show you the exact future date. For example, adding 12 weeks to January 1, 2026 gives March 26, 2026.

Is there a limit to how far apart the dates can be?

There is no practical limit. The calculator can handle dates spanning thousands of years. However, for very old dates, keep in mind that the Gregorian calendar was only adopted gradually around the world from 1582 onward, so historical accuracy may vary for earlier dates.

How accurate is the years-months-days breakdown?

The breakdown is exact for the Gregorian calendar. The Luxon library computes full years first, then remaining full months, then leftover days. Each step uses the actual calendar length of the relevant months, so there is no rounding or approximation involved.

Can I use negative numbers in the Add/Subtract mode?

You do not need to. Simply switch the operation from 'Add (+)' to 'Subtract (−)' and enter a positive number. The calculator will move the date backward in time by the specified amount.

Sources and References

ISO 8601 Date and Time Format — International standard defining date representation formats used in this calculator.
Gregorian Calendar – Encyclopaedia Britannica — Reference for the calendar system underpinning all date arithmetic.
Luxon Documentation — Date-time library documentation for the calendar-aware math used in calculations.
ECMAScript Internationalization API — Standard underlying JavaScript's date formatting capabilities.
Schema.org WebPage Specification — Structured data schema used for this page's metadata.