Yes, that’s correct.
While national return tickets are good to travel from A to B and from B to A, return tickets in fare networks automatically convert into a day pass. Tickets which use the zone-based system entitle you to travel on any eligible public transportation within all included zones throughout the ticket’s entire validity period. In other words, you can travel as many times you like and this not only on the stretch you originally got the ticket for yet in all the zones mentioned on the ticket.
Usually, a day pass is valid from today 00:00 until tomorrow 05:00 (AM), no matter the time it was bought. In the Zurich fare network (ZVV), however, day passes are valid exactly 24 hours, counted from the time of purchase.
Does my explanation resolve your confusion?