I trust you understand that DATEDIFF returns the count of the specified datepart boundaries crossed between the specified startdate and enddate.
eg...even though the start and end times below are only 2 minutes apart, datediff for hours returns 1
DECLARE @start datetime = '2016-05-30 09:59:00.000',
@end datetime = '2016-05-30 10:01:00.000'
SELECT DATEDIFF(hour,@start,@end)
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