how to calculate business hours between days in sqlserver 2008r2

SET DATEFIRST 7

declare @start as date
declare @end as date

set @start = '17/12/2013'
set @end = '17/01/2014'

declare @start_hr as time
declare @end_hr as time

set @start_hr = '9:00'
set @end_hr = '17:00'

declare @workdayscount as int

set @workdayscount = 0

while @start < @end
begin
	set @workdayscount = @workdayscount + case when datepart(weekday, @start) in (1, 2, 3, 4, 5) then 1 else 0 end
		
	set @start = dateadd(day, 1, @start)
end

select @workdayscount
select @workdayscount * datediff(hour, @start_hr, @end_hr)