Hi @bobo_97
Thank you for your question.
It is up to you: you can purchase it on any given day and have it valid from that same day, or place the first validity day up to two months in the future. You can let our colleagues at the SBB counter know from when exactly you wish to start the half fare travelcard, or you can set the date whie purchasing it online.
I wish you a nice day
EleonoraS