Today I am going to show you one way to calculate a decimal year. This is not a easy as it sounds because some years (leap years) include a leap day so the number of days in a year is not consistent.
The longest defined consistent time period there is, is a week. A week is seven days or 168 hours. However, a week can be defined to have different start weekday in different regions of the world. Enough said about that.
First, set up a test environment by declaring and setting two datetime variables like this
declare @f datetime
select @f = '03/01/2004', @t = '02/28/2012'
What we need now is an algorithm that for every day in a leap year is divided by 366 days and for every day in a non-leap year is divided by 365.
WHEN y = 0 THEN 1.0E * (t - f) / d
WHEN Number = 0 THEN 1.0E * (d - f + 1) / d
WHEN Number = y THEN 1.0E * t / d
END) AS DECIMAL(10, 2))
DATEDIFF(YEAR, @f, @t) AS y,
CASE ISDATE(10000 * Number + 10000 * DATEPART(YEAR, @f) + 229)
WHEN 0 THEN 365
END AS d,
DATEPART(DAYOFYEAR, @f) AS f,
DATEPART(DAYOFYEAR, @t) AS t
WHERE Type = 'P'
AND Number <= DATEDIFF(YEAR, @f, @t)
) AS d
In my example above, I am using the master..spt_values table which for SQL Server 2005 and later has 2048 records (2048 years) and for SQL Server 2000 and earlier has 256 records (256 years). You can easily replace the master..spt_values table with a number tally table of your own. Just make sure it is zero-based or you will have to edit the suggestion above.
Here is a link how DATEDIFF works