Originally Posted by LarryJ

A layover must be a minimum of ten hours. Without an intervening 10-hour layover, the pilots would remain on duty all night. The flight duty period limit for an evening report (5pm-9:59pm, with two flight segments) is 12 hours and starts one-hour before schedule departure time.

To be legal, you either have to fit the entire trip within that 12-hour flight duty period or you have to have a minimum of a 10-hour layover to break it into two flight duty periods. So, for an 8:00pm departure the night before, you'd have to be scheduled to complete the return no later than 7:00am the following morning. You must be estimated to arrive within those constraints at the beginning of the takeoff roll on each flight which is why you can sometimes taxi out but, after a delay, have to return back to the gate due to crew legalities.

https://www.law.cornell.edu/cfr/text/14/part-117
Table B is the applicable duty time limits.