After verification, the WHERE clause need to be like this so to include in the record set the minimum and maximum values:

DECLARE @Start DATETIME
DECLARE @End DATETIME
DECLARE @Start2 DATETIME
DECLARE @End2 DATETIME
 
SET @Start='2014-01-23 09:15:00'
SET @End='2014-01-29 13:16:00'
SET @Start2='2014-01-23 08:00:00'
SET @End2='2014-01-29 17:00:00'
 
SELECT SUM(Temp.SecondValue)/3600 AS HourValue
FROM (SELECT DATEDIFF(SECOND,
 CASE WHEN BusinessTime.Start>@Start THEN BusinessTime.Start ELSE @Start END,
 CASE WHEN BusinessTime.[End]<@End THEN BusinessTime.[End] ELSE @End END) AS SecondValue
 FROM BusinessTime
 WHERE BusinessTime.NoProvince=84 AND
  BusinessTime.Start>=@Start2 AND BusinessTime.Start<=@End2 AND
  BusinessTime.[End]>=@Start2 AND BusinessTime.[End]<=@End2) Temp

However, the /3600 rounds the value. I need to find a way to collect the decimals.