Hi - Just wondering if we get a return ticket from Zurich to Lucerne, do we need to specify the return train time? If we want to take earlier/later train, would it be a problem? Thank you.
There really are no return tickets, its just 2 one-way tickets and you can take any train you like as the tickets are valid on any train on that route for the day.
Depending on what you are doing, you may want to buy a Saver Day Pass now for your travel date. You can take any transport mode anywhere in Switzerland for the day in the GA travel area. That includes scheduled lake boats, and even the Rigi Bahn to the Rigi Kulm and back. Cost should be 52 CHF per person if you by now for a date later this summer. Since a full fare train ticket from Zurich to Luzern is 27 CHF each way, you will save just on that.
https://www.sbb.ch/en/tickets-offers/tickets/day-passes/saver-day-pass.html
Nowadays the purchasing process indeed starts with the selection of a train. And that may create the impression that you are buying a ticket for that particular train. That is however not how this works.
In the past it was simpler: You would enter origin, destination and date, and just got a ticket for the route you selected, valid on the day you entered.
And that is how tickets work: Train tickets are for a route, not for a train.