Ho To Convert Varchar Datatype Into Datetime

declare @a varchar(10)
select @a= shiftstarttime from o_parameter
print @a

declare @b varchar(10)
select @b= shiftendtime from o_parameter
print @b


declare @dt varchar(20)
set @dt=Left(getdate(),12)

print @dt

declare @dt1 varchar(20)
set @dt1=Left(dateadd(dd, 1,getdate()),12)
print @dt1

declare @dt3 varchar(20)

set @dt3= @dt1 + space(0) + @a

print @dt3

declare @dt4 varchar(20)

set @dt4= @dt + space(0) + @b

print @dt4




output of above script is as fallow
09.30am
06.30pm
Aug 22 2007
Aug 23 2007
Aug 23 2007 09.30am
Aug 22 2007 06.30pm

now i want to convert @dt3 and @dt4 into datetime .

because i wnat to calculate datedifference in hours by using this function
SET @in_hour=DATEDIFF (HH ,@D1,@D2)

where @D1 and @D2 are datetime parameters.

but how can i get @dt3 and @dt4 into datetime so i can pass it to DATEDIFF() function.

so plz Guide me. or if u know how to write it then plz write here

i already use cast for it but it doesn't work.

so plz reply urgently.