Saw this in another thread so reposting for greater visibility:
Time slots for Acropolis tickets are now RESTRICTIVE - meaning that your ticket is valid for the time of the reservation only. Previously the ticket was valid for the whole day.
From 15th June 2023, the compulsory selection of a visit zone in the Acropolis Archaeological Site for the purchase of tickets online will be implemented.
Visitors can enter the Acropolis Archaeological Site strictly at the time they selected when they issued their ticket, according to the terms and conditions of use of the e-ticket.
Visitors to the Archaeological Site are kindly requested to be at the site 30 minutes before the zone they have chosen, for the best possible service.
As I understand it this only applies to the Acropolis and not adjacent sites like the Agora, Theatre of Dionysus etc