Description of datediff

{ DATEDIFF | TIMESTAMPDIFF } 
    ( { YEAR
	 | MONTH
	 | DAY
	 | WEEK
	 | QUARTER
	 | HOUR
	 | MINUTE
	 | SECOND
	 | MILLISECOND
	 | MICROSECOND
	 | NANOSECOND	
      } , start_datetime , end_datetime [ , start_of_period ]		 
   )