Free 100% In-Browser Tool

Date Calculator

Add or subtract days, weeks, months, and years from any starting date with precise calendar and leap year adjustments.

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How to Use the Date Calculator – Add or Subtract Calendar Days

1Select your starting calendar date using the date picker.
2Choose whether you want to Add (+) or Subtract (-) time.
3Enter the duration in years, months, weeks, and days.
4View the resulting target calendar date, day of the week, and leap year status.

Formula & How It Works

Calendar Arithmetic & Month-End Clamping Method

Target Date = Start Date ± (Years, Months) with Month Clamping ± (Weeks × 7 + Days)

Calendar calculation adjusts the year, month, and day components while respecting variable month lengths (28, 29, 30, or 31 days). If adding months causes a day overflow (e.g. January 31 + 1 month), the calculation clamps to the final day of February (Feb 28 or Feb 29 in leap years) before adding week and day offsets.

Worked Examples

Standard 90-Day Probation Window

Input: Start: 2026-01-01, Add: 90 days

Explanation: Adding 90 calendar days to Jan 1 advances through 30 remaining days in Jan, 28 in Feb, 31 in Mar, landing on April 1.

Result: April 1, 2026 (Wednesday)

Leap Year Month Addition

Input: Start: 2024-01-31, Add: 1 month

Explanation: Since 2024 was a leap year, adding 1 month to Jan 31 clamps to the final leap day of February.

Result: February 29, 2024 (Thursday)

Six-Month Contract Expiration

Input: Start: 2026-06-15, Add: 6 months

Explanation: Adding 6 months advances directly to Dec 15 of the same calendar year.

Result: December 15, 2026 (Tuesday)

Historical Event Lookback

Input: Start: 2026-11-01, Subtract: 5 years, 2 weeks

Explanation: Subtracting 5 years yields Nov 1, 2021, and subtracting 14 days lands on Oct 18, 2021.

Result: October 18, 2021 (Monday)

Frequently Asked Questions

How does the date calculator handle leap years?▼

The algorithm recognizes leap years (years divisible by 4, except century years not divisible by 400). February accurately accounts for 29 days in leap years.

What happens when adding 1 month to March 31?▼

Since April has only 30 days, the calculation clamps to April 30, preventing date overflow into May.

Does this calculator use my local time zone?▼

Yes, the tool detects your browser's local time zone via the Internationalization API and displays it at the top of the interface.

Can I subtract dates backward in time?▼

Yes, select the 'Subtract (-)' button to calculate historical dates in the past.

How many days are in 6 weeks?▼

There are exactly 42 calendar days in 6 weeks (6 × 7 = 42 days).

Are public holidays accounted for in calendar date addition?▼

This tool calculates continuous astronomical calendar days. For business-only working days, use our Days Between Dates tool.

Last reviewed & updated: 2026-11-01

Free & Open Access