Here's my pseudo-code expanded a bit to accommodate the start and end date bits:
declare @StartDate datetime
set @EndDate = dateadd(hour, 8, DATEADD(dd, 0, DATEDIFF(dd, 0, DATEADD(HOUR, - 8, getdate()))))
set @StartDate = dateadd(hour, - 73, @EndDate)
where [convert varchar date and time to datetime] between @StartDate
That's how it works in T-SQL. Just need to convert that to Oracle.
--Edit: fixed typo.
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