Hello Quoiberau97
Welcome to the SBB Community.
- The Day Pass for the Half Fare Card is only available to customers holding the (Swiss) Half Fare Card in addition. It cannot be used by customers not having this card.
The cost of CHF 78 for 2nd class or CHF 128 for 1st class is fixed; thus, it costs the same, no matter the time of purchase. And, it can be cancelled any time before the validity date.
https://www.sbb.ch/en/tickets-offers/tickets/day-passes/day-pass-for-the-half-fare-travelcard.html
- The Saver Day Pass is available to anyone; the ½ fare is available to customers holding the (Swiss) Half Fare Card, the full fare is available to customers without the (Swiss) Half Fare Card.
The cost is variable, the price raises the nearer the travel date and the higher the demand.; and, it can even sell out or the cost can be higher than for a Day Pass for the Half Fare Card. Furthermore, it is not cancellable and not refundable. In a nutshell, the purchase of a Saver Day Pass is like gambling. If you are lucky, you travel for little money.
https://www.sbb.ch/en/billette-angebote/billette/tageskarten/spartageskarte.html
→ What concerns the range of validity, the Day Pass for the Half Fare Card and the Saver Day Pass indeed offer the same amenities.
Does that answer your question?
Kind regards,
Roland