Subtract two time values in ms sql server 2008

You can use DATEDIFF():

SELECT DATEDIFF(Day, startDate, endDate)
  FROM table

SELECT DATEDIFF(Second, date, GETDATE())
  FROM table

DECLARE @END TIME = '16:00:00.0000000' ,    
     @START TIME = '01:00:00.0000000'
     SELECT convert(TIME,dateadd(ms,DateDiff(ss, @START, @END )*1000,0),114)

following expression works for me

declare @starttime Time, @endtime Time
set @starttime='18:45'
set @endtime='22:45'
select DATEDIFF(HH,@starttime, @endtime)

output: 4