The prime definition is strictly for television, and refers to when the programs air during the day – primetime is considered anytime after 6pm and before midnight (for example 7p-11p). To change it for a particular order, go to Prime, and select Prime Definition in the schedule builder. Use the up & down arrows the change the start time and the end time.
On the Prime Type tab, decide whether the calculation is based on Cost, Audience or Rating