Date Calculator

The Formula

Add or subtract a combination of years, months, weeks, and days from a start date, with precise handling of calendar boundaries. For example, adding 1 month to January 31 yields February 28 (or 29 in a leap year), not March 3 — the calculator intelligently clamps to the last valid day of the target month. When using business days mode, weekends (Saturday and Sunday) are skipped and only weekdays count.

Variable Definitions

How to Use This Calculator

1. 1

   Enter a start date in YYYY-MM-DD format (e.g., 2024-01-15 for January 15, 2024).

2. 2

   Choose whether to add or subtract time from the start date using the radio buttons.

3. 3

   Enter the number of years, months, weeks, and/or days to adjust by — any combination is supported.

4. 4

   Select whether to count calendar days (including weekends) or business days only (Monday-Friday).

5. 5

   Review the resulting date, along with any boundary adjustments the calculator applied.

Date arithmetic adds or subtracts years, months, weeks, and days, with intelligent month-end clamping.

Understanding the Concept

This calculator lets you add or subtract years, months, weeks, and days from any date. Date arithmetic is more complex than it seems: months have varying lengths (28 to 31 days), leap years add an extra day in February, and adding a month to a late date like January 31 requires careful clamping to avoid rolling into an invalid date. The calculator handles all these edge cases automatically. Business day mode is particularly useful for project planning, contract deadlines, and legal timelines where weekends don't count toward progress.

Frequently Asked Questions

Sources & References