Find Difference In Time
First of all I'm not sure why you are dividing by 3600 .. that would return you 1/3600th of a second.

Second you are over thinking things a bit.  Try this:

CREATE TABLE #NotDope
(
  date1 datetime
)

INSERT INTO #NotDope (date1) Values
('2016-01-04 08:08:47.000'),
('2016-01-04 08:25:59.000'),
('2016-01-04 08:27:47.000'),
('2016-01-04 08:00:00.000'),
('2016-01-04 07:55:00.000')


SELECT date1,
    CASE WHEN CAST(date1 AS Time) &gt; '08:00:00.000' THEN
        datediff(second, '08:00:00', CAST([date1] AS time))
    ELSE 0 END AS SecondsLate
FROM #NotDope


All you need to do is cast your date as a time data type to get rid of the date part.