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Add / Subtract Date Calculator

Move a calendar date by years, months, weeks and days with explicit month-end handling.

Your numbers

Make it your own
Updates as you type
Resulting calendar date
2026-02-28

Calculated from your inputs · rounded for display

WeekdaySaturday

Calendar arithmetic in UTC. Month-end clamping means adding and subtracting months are not always inverse operations.

Your calculation, step by step

No black boxes
  1. Start at 2026-01-31.
  2. Apply signed years and months together; clamp the day to the last valid day of the destination month.
  3. Then apply signed weeks and days. Result: 2026-02-28.

How to use this calculator

  1. Enter your own values in the labeled fields. Check the units and assumptions before comparing results. Only the fields needed for your selected calculation mode are shown.
  2. Your answer updates automatically. Review the calculation steps to see exactly how it was calculated.
  3. Copy, share, save, or download your result. Saved inputs stay on this device.

How the calculation works

Apply signed years and months with month-end clamping, then signed weeks and days

Calendar months have different lengths. This tool moves years and months together, clamping the original day to the last valid day of the destination month. It then applies weeks and days. This documented convention avoids JavaScript date overflow into an unexpected later month, but month addition and subtraction need not be inverses.

A worked example

January 31, 2026 plus one calendar month becomes February 28, 2026 under the clamping convention.

Reading your result

Use the main result together with its units and supporting figures. Display values are rounded; calculations keep their underlying precision. A change in inputs or assumptions can change the answer. Compare the step-by-step working with your intended use before relying on an estimate.

Frequently asked questions

Is a month treated as 30 days?

No. It is a calendar-month operation.

Are weekends skipped?

No. All calendar dates are included.

Sources & methodology

This calculator uses the mathematical identity and assumptions stated above. No current rates, prices, or external data are loaded. Conversion tools use international unit definitions.

Formula review: October 24, 2026. Read our methodology.