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Anniversary Calculator

Calculate date differences, milestones, and more in one place.

Inputs

Calculate day milestones and yearly anniversaries from a start date.

This option affects duration only. The D-day display is unchanged.
Your dates are not sent to or stored on the server.
CALCULATION RESULT

How to use

  1. 01

    Choose the starting date

    Enter the event’s first day and confirm the calendar date.

  2. 02

    Calculate milestones

    Review the offered day milestones and yearly anniversaries. The starting date is day one for day-count milestones.

  3. 03

    Check leap-year cases

    Verify February dates and the counting convention before adding an event to your calendar.

Frequently asked questions

Why is day 100 only 99 days after the start?

The starting date counts as day one. The 100th day is therefore 99 days later.

What happens to February 29 anniversaries?

Yearly anniversaries use February 28 in non-leap years under this tool’s existing rule.

Are holiday calendars included?

No. The results use Gregorian calendar dates without a public-holiday calendar.

Important information

Tool files and calculation inputs are processed in your browser, without uploading them to a processing server.

Keep original files and reopen downloaded results before replacing a working copy.

Tested example

Input and settings

Treat 2024-01-01 as day one

Observed result

Day 100 is 2024-04-09

What to check

The 100th day differs from 100 days later. Record the counting convention.

Examples use synthetic, non-personal test material. Results apply to the inputs and settings shown; different files and devices can produce different sizes, timing and appearance.