Calculate an anniversary date, start date, or years elapsed by entering any two values and finding the missing date or years total.
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Anniversary Formula
The anniversary calculator uses calendar-year addition or subtraction when you enter a start date, a number of years, and an anniversary date. Enter any two values to calculate the third.
For the elapsed-years formula, the adjustment value is:
- AD = anniversary date
- SD = start date
- Y = number of full calendar years
- A = adjustment for whether the anniversary month and day has occurred yet in the anniversary year
If you enter the start date and number of years, the calculator adds that many calendar years to find the anniversary date.
If you enter the anniversary date and number of years, the calculator subtracts that many calendar years to find the start date.
If you enter the start date and anniversary date, the calculator finds the number of full years elapsed. If the anniversary date is before the start month and day in that year, it subtracts 1 from the simple year difference.
Common Anniversary Year Names
Some anniversaries have traditional names. The table below shows common milestone years.
| Years | Common Anniversary Name |
|---|---|
| 1 | First anniversary |
| 5 | Fifth anniversary |
| 10 | Tenth anniversary |
| 25 | Silver anniversary |
| 50 | Golden anniversary |
| 60 | Diamond anniversary |
How Full Years Are Counted
When calculating the number of years between two dates, the calculator counts full years only.
| Start Date | Anniversary Date | Full Years | Reason |
|---|---|---|---|
| 2020-06-15 | 2025-06-15 | 5 | The month and day match. |
| 2020-06-15 | 2025-06-14 | 4 | The anniversary day has not occurred yet in 2025. |
| 2020-06-15 | 2025-07-01 | 5 | The anniversary day has already passed in 2025. |
Example
Example 1: Find an anniversary date
You started on 2016-04-20 and want to find the 10-year anniversary.
Anniversary date = 2026-04-20
Example 2: Find the number of full years
You started on 2018-09-10 and the anniversary date is 2024-09-09.
The month and day, September 9, is before September 10, so subtract 1.
Number of full years = 5
FAQ
Does the calculator count full years or rounded years?
It counts full calendar years. For example, from 2020-05-10 to 2025-05-09 is 4 full years, not 5, because the fifth anniversary has not happened yet.
What happens if the start date is February 29?
If you add years to February 29 and the target year is not a leap year, the date may shift to March 1. This follows the date behavior used by JavaScript calendar calculations.
Can the anniversary date be before the start date?
Not when calculating a non-negative number of years. The anniversary date must be on or after the start date if you want the calculator to return the number of elapsed years.
