Execute the following Microsoft SQL Server T-SQL example script in Management Studio Query Editor to calculate the difference between two dates in years, months and days.

-- SQL calculate age - Calculating age in years from DOB - Age calculation

-- SQL calculate length of service / employment in years, months, days

-- SQL calculating duration / delta / difference between two dates

-- T-SQL datediff datetime function - TSQL dateadd datetime function

USE AdventureWorks;

DECLARE  @EndDate             DATETIME,

         @Anniversary         DATETIME,

         @StartDate           DATETIME,

         @YYDelta             INT,

         @MMDelta             INT,

         @DDDelta             INT

SET @StartDate = '2000-01-01'

SET @EndDate = Getdate()

SET @Anniversary =

      Dateadd(yy,Datediff(yy,@StartDate,@EndDate),@StartDate)

SET @YYDelta = Datediff(yy,@StartDate,@EndDate) -

(CASE

   WHEN @Anniversary > @EndDate THEN 1  ELSE 0   END)

SET @MMDelta = Month(@EndDate - @Anniversary) - 1

SET @DDDelta = Day(@EndDate - @Anniversary) - 1

SELECT Years  = @YYDelta,

       Months = @MMDelta,

       [Days] = @DDDelta

GO

/* Results

Years       Months      Days

9           1           19

*/

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Related link:

http://sqlusa.com/bestpractices/datetimeconversion/