Date Arithmetic & Calculation
How It Works
This calculator performs precise, deterministic date arithmetic entirely in your browser. There are three modes:
- Days Between — Enter any two dates to get exact days, months, years, weeks, business days, and weekend count.
- Add / Subtract — Add or subtract days, weeks, months, or years from any date to find the resulting date.
- Business Days — Count business days (Mon-Fri) between two dates, or calculate the date after adding N business days.
Why AI Gets Date Math Wrong
Large language models like ChatGPT, Claude, and Gemini frequently make errors in date arithmetic because they:
- Miscalculate month boundaries (e.g., Jan 31 + 1 month should be Feb 28)
- Forget or misapply leap year rules (year 2000 is leap, 1900 is not)
- Give different answers to the same question at different times
- Cannot reliably count business days when spanning weeks or months
This tool uses JavaScript's native Date object with deterministic algorithms — you get the same correct answer every time.
Frequently Asked Questions
How does this calculator count months between dates?
We use full calendar months. For example, March 15 to August 20 is 5 months and 5 days (March→April→May→June→July→August). Partial months are calculated by actual day counts, not average month length.
Does it account for leap years?
Yes. The calculator uses JavaScript's built-in Date object which correctly handles all leap year rules including century years (year 2000 was a leap year, 1900 was not, 2100 will not be).
What counts as a business day?
Business days are Monday through Friday, excluding Saturdays and Sundays. This calculator does not account for public holidays — for holiday-specific calculations, consult a local calendar.
Is my data private?
100%. All date calculations happen locally in your browser. No data is sent to any server. Your dates never leave your device.
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