I always assumed that the time buckets in the time table for operation modes were fixed to one hour. But recently, while teaching new Basis administrators, I realized the minimum time frame is 15 minutes. This setting provides more flexibility while assigning operation modes to the 24 hour period.
Call transaction RZ04 and use menu Operation mode -> timetable and edit the normal or exception time table.
Once in the time table view use menu Edit -> Time period and choose the length of the time bucket you want to mange for the assignment of operation modes.