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ZVV (Zurich regional fare network) tickets are available 3 months in advance. No resident would ever THINK of buying such a ticket that far in advance, as it will be still available (in unlimited numbers) at the same price until 1 minute before departure.
As Krist already mentioned above, you do not “book” a particular train or seat (excepted a few ‘panoramic’ tourist trains). Like around many other European cities, you get the correct zone ticket suggested (with a specific validity period of 1-2 hours for ‘single’ journeys, 24h or day pass for ‘return’ trips) for the connection you’ve selected in the timetable. Such a zone ticket allows free travel back and forth in any type of public transportation within the listed zones during that period.