String &> Datetime Conversation Problem

hi,
i have a simple code here but i always get same error.
my code is :

declare @date datetime
declare @dates varchar(30)
declare @day varchar(2)
declare @month varchar(2)
declare @year varchar(4)


set @date = (select dateadd(day,0,getdate()))
set @day = (select datepart(day,@date))
set @month = (select datepart(month,@date))
set @year = (select datepart(year,@date))
set @tarih = (select '''' + @year + '-' + @month + '-' + @day + ' 00:00:00.000''' )

if @month < 10
begin
set @month = '0' + @month
end
if @day < 10
begin
set @day = '0' + @day
end
-------------IT GIVES ERROR HERE
set @dateS = (select '''' + @year + '-' + @month + '-' + @day + ' 00:00:00.000''' )

---
well ,
when i try this >
set @date (which is datetime format ) = '2008-03-22 00:00:00.000'
it works(manually),
but i cant dynamically bind my string to datetime formatted field.

then i tried,
set @dates = (select '''' + @year + @month + @day + '''' )
set @date = (select convert(datetime,@dates)


and it DOESNT work too,
what to do ?

MS BLESS US