Even though you start the purchase process of a ticket by selecting a departure time this does not imply that your ticket will be tied to that departure time.
You start by selecting a trip, because that is how the system determines what route you intend to travel, and at what time you intend to travel. It will then offer you multiple options. It might offer you a supersaver ticket, which is indeed timed, but it will usually also offer you a point-to-point ticket, which is not timed.
Only for short distances you should pay attention, as such tickets often are calculated using a local zone based tariff, and those tickets are typically not valid all day.
Anyway for short, trivial trips there is no need to buy tickets in advance. You have up to the moment of departure to buy your ticket.
