SQL Function to convert date to work week

create function [dbo].[convertToWW]
(@mydate varchar(12))
returns varchar(10)
as begin
declare @ww varchar(12)
set @ww = cast(((select DATEDIFF(day, '1/1/2007', @mydate))/7%52+1) as varchar(2)) + '.' + cast(((select DATEDIFF(day, '1/1/2007', @mydate))%7+1) as varchar(2))+''''+ cast(((select DATEDIFF(day, '1/1/2007', @mydate))/7/52+2007) as varchar(4))
return @ww
end
go

select dbo.convertToWW('5/5/2007')