declare @d datetime = '09/08/2012 09:14:55'

set @d = SETDATEPARTORSOMETHINGLIKETHAT (hour, @d, 23)

DECLARE @TargetHour TINYINT = 23;

DECLARE @d datetime = '09/08/2012 09:14:55';
SET @d = DATEADD(HOUR, @TargetHour - DATEPART(HOUR, @d), @d);
SELECT @d;

2012-09-08 23:14:55.000

DECLARE @d datetime = '2012-09-08T09:14:55';