Anniversary Calculator
Enter a start date and an as-of date to see completed anniversaries and upcoming milestone dates.
A QUICK WALKTHROUGH
How to use this tool
- Enter a valid start date and as-of date.
- Select Calculate anniversaries.
- Review the completed years, total calendar days, and milestone dates.
Date-only calendar arithmetic
Dates are parsed as calendar dates at UTC midnight. No local time zone, clock time, daylight saving change, or elapsed-hour approximation affects the result.
Leap-day anniversaries
When a start date is February 29 and the target year is not a leap year, the anniversary is February 28. Calendar-day totals include every leap day between the dates.
GOOD TO KNOW
Common questions
What does the calculator count?
It shows completed calendar years, the exact number of calendar days, and milestone dates for the next ten anniversaries.
Is February 29 supported?
Yes. Leap days are validated, counted in day totals, and mapped to February 28 for non-leap-year anniversaries.
Are time zones used?
No. Inputs are date-only values and are interpreted with timezone-free calendar rules.
Is my date uploaded?
No. The calculation runs locally in your browser.