Syntax:
 DATEDIFF ( datepart , startdate , enddate ) Following is an example for the DateDiff()
DECLARE @startdate datetime ='2011-11-14 02:00:06.957';
 DECLARE @enddate datetime = '2011-11-16 05:00:15.490';
SELECT DATEDIFF(day, @startdate, @enddate) as dayDifference,DATEDIFF(HOUR, @startdate, @enddate)as hourDifference 
OutputdayDifference    hourDifference   
  2                     51  
 
Similarly the following are the available
 datepart Options on the DateDiff() Function.
year(or yy or yyyy)
quarter(or qq or q)
month (or mm or m )
dayofyear(or dy or y )
day (or dd, d )
week(or wk or ww)
hour(or hh)
minute(or mi or n)
second(or ss or s )
millisecond(or ms)
microsecond(or mcs) 
nanosecond(or ns) 
TZoffset(or tz)
ISO_WEEK(or isowk or isoww)
