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Date Calculator

Find the difference between two dates, or add and subtract days, weeks, months, and years from any date.

Result

About the Date Calculator

The Oneyfy Date Calculator has two modes: Find Difference calculates the exact span between two dates in multiple units, while Add/Subtract lets you find a resulting date after applying a time offset.

How to Use

  1. Choose a mode: Find Difference or Add / Subtract.
  2. For Find Difference: enter a start date and an end date.
  3. For Add/Subtract: enter a base date, choose to add or subtract, enter an amount, and pick the unit (days, weeks, months, years).
  4. Click Calculate to see the result.

How It Works

Date differences are computed using JavaScript's built-in Date object. Millisecond differences are converted to days, then to weeks, months, and years. Month and year addition/subtraction correctly handles calendar roll-over (e.g., adding 1 month to January 31 gives February 28/29).

Example

Start: January 1, 2024 · End: October 15, 2024 → Difference: 288 days (9 months, 14 days, or ~41 weeks).

Base: March 31, 2024 · Add 2 months → Result: May 31, 2024.

Frequently Asked Questions

No. The difference is calculated exclusive of the start date by default — the same way you count elapsed days (from day 0 to day N = N days have passed).
Months are calculated by counting calendar month boundaries crossed, then adjusting for the remaining days. This gives a natural reading of "X months and Y days."
Yes. The calculator will show a negative difference (i.e., the end date is before the start date) and label it accordingly.