Even not talking that traffic jams and train delays happen most days of the year in the Rotterdam region.
The idea to being forced to arrive on a half hour slot is inherently against the idea of a relaxing family day in the zoo. A zoo is not the same type of entertainment as a cinema or a concert. It is one of more nonsense management decisions in the zoo world I know of.
I guess Blijdorp zoo wants to focus on being a local zoo for Rotterdam citizens who can travel by foot or a bicycle.
I actually learned tonight that the time slots at Rotterdam only 15 minutes rather than half an hour. That is really narrow, even for people who live in or near Rotterdam. With red lights or traffic you might not even make that on bicycle. With strict enforcement I don't see a lot of people getting into Rotterdam Zoo, they must be implementing this somewhat loosely ...