Hello swissbio
A one way ticket from A to B is good for one journey from A to B. If you buy a single and return ticket, it is good for two journeys, one from A to B and one from B to A, on the same day. You cannot take the same trip multiple times as with zone tickets. You are allowed to break your journey as often as you want on the way from A to B / B to A but no backtracking.
If you take a trip from A to B on Monday, and B to A on Friday, you must buy two single tickets for each direction as a single and return ticket is not valid multiple days.
A day is considered from 00:00 today until 05:00 in the morning of the next day. Within that time frame you can complete the journey as indicated on your ticket. But, once only!
Does that answer your question?
Kind regards,
Roland