Datediff is the function for you if you want to calculate different between 2 dates, and make sure you supply the unit you would like to measure :
select ‘Year’, datediff(yyyy, ‘2005-01-22 10:20:00′, ‘2008-01-22 16:20:00′)
=> 3
select ‘Month’, datediff(m, ‘2008-01-22 10:20:00′, ‘2008-12-22 16:20:00′)
=> 11
select ‘Day’, datediff(d, ‘2008-01-22 10:20:00′, ‘2008-01-29 16:20:00′)
=> 7
select ‘Hour’, datediff(hh, ‘2008-01-22 10:20:00′, ‘2008-01-22 16:20:00′)
=> 6
select ‘Minute’, datediff(mi, ‘2008-01-22 10:20:00′, ‘2008-01-22 16:20:00′)
=> 360
select ‘Second’, datediff(s, ‘2008-01-22 10:20:00′, ‘2008-01-22 10:20:30′)
=> 30