Write a sql server function to find the Start and End date of given ISO week

I faced a scenario where I need to get the first and last date of given ISO Week

To know about the concept of ISO week of the year please refer the following URL:
http://en.wikipedia.org/wiki/ISO_week_date

Here the function is as follows:

CREATE FUNCTION [dbo].[FncGetISOStartEndDateGivenWeekNo]
(
	@GetFor VARCHAR(25),
	@WeekNo INT,	
	@Year INT = NULL
)
RETURNS DATETIME
AS
BEGIN
	DECLARE @DTSTARTENDDATE DATETIME	
	DECLARE @DayofYear AS INT   	
	SELECT @DayofYear = (@WeekNo *7)-1
	DECLARE @DATEDIFF AS INT
	DECLARE @FLAG AS BIT
	
	IF ((@Year = NULL) OR (@YEAR IS NULL))
	BEGIN
		SET @Year = YEAR(GETDATE())		
	END
	
	SET @FLAG = 0
	IF (@GetFor = 'TODATE')
	BEGIN
		SET @DATEDIFF = 0
		SET @FLAG = 1
	END
	ELSE IF (@GetFor = 'FROMDATE')
	BEGIN
		SET @DATEDIFF = 6
		SET @FLAG = 1
	END
	IF (@FLAG = 1)
	BEGIN
	
	   DECLARE @date AS DATETIME
	   DECLARE @stDateofCurYear DATETIME
	   SET @date = CAST((CAST(@Year AS VARCHAR(5))+'0101') AS DATETIME)	   	   
	   
	   --Region For getting the First Day and First Week of the Current year
	   
	   SELECT @stDateofCurYear = OuterQry.CurrYrStart FROM (			
				SELECT
						-- First day of first week of current year						
						CurrYrStart =	DATEADD(DAY, (DATEDIFF(DAY, '17530101', InnerQry.Jan4th) / 7) * 7, '17530101')
		
				FROM
				(
						SELECT
							--Get the January 4th Day of input date's year
							Jan4th	= 
							DATEADD(dd,3,DATEADD(yy,DATEDIFF(yy,0,@date),0))
				) InnerQry 
		) as OuterQry
		--Endregion 	   	   	   			
   
    SELECT @DTSTARTENDDATE = DATEADD(DAY, (@DayofYear-@DATEDIFF), @stDateofCurYear)			    	    
	
	END
	ELSE
	BEGIN
	   SET @DTSTARTENDDATE = NULL	  
	END
	RETURN @DTSTARTENDDATE
END

Run for Testing
---------------------

select dbo.FncGetISOStartEndDateGivenWeekNo('FROMDATE',45,2012) -- 2012-11-05 00:00:00.000
select dbo.FncGetISOStartEndDateGivenWeekNo('TODATE',45,2012) -- 2012-11-11 00:00:00.000
select dbo.FncGetISOStartEndDateGivenWeekNo('FROMTODATE',45,2012) -- Print NULL

First I am getting the number of days with respect to given week Here 45.

select (45*7) -1 -- Get the Number of days and sub by 1. Here 314

In Inner query I am finding the 4th Day of given year. If given year is 2012 then 04-Jan-2012

To find the First day of First ISO Week, We need to pass 4th date to outer query.

The outer query works as follows:

--- No of days

select DATEDIFF(DAY, '17530101', '20120104') 

-- No of weeks

select DATEDIFF(DAY, '17530101', '20120104')/7 

--Get the StartDay of ISO Week 1

select DATEADD(DAY, 13514 * 7, '17530101') 

For FromDate : --For FromDate Subtracting By 6

select DATEADD(DAY, (314-6), '20120102')

To get Last Date: -- ToDate Subtracting By 0

select DATEADD(DAY, (314-0), '20120102')