UDF to Get the DD Part of the last Date of Month

-- =============================================
A UDF to get the DD part of the last date of the month according to the date passed into its parameter
-- =============================================
CREATE FUNCTION [dbo].[fn_DD_OF_Last_Date_OF_Month]( @myDate as datetime)
RETURNS INT
AS
BEGIN

DECLARE @return AS INT
SET @return =0
Declare @myDate2 datetime


SET @mydate2=dateadd(m,1,@myDate)-day(dateadd(m,1,@myDate))

SET @return= cast (datepart(dd,@myDate2) as int)

RETURN @return
END

The following query is to test the UDF:

select [dbo].[fn_DD_OF_Last_Date_OF_Month]( '03/13/2005')