You do not buy the pass with an open date. When you purchase the pass you indicate on which date you want it to start. You probably neglected to change the proposed start date, which defaults to now.
So the pass starts on the day you indicate. There is nothing that “detects” that you are using the pass, so having it activate only when you use it is not possible.
Contact SBB to get a refund.
There is btw. no need to buy this pass so far in advance. You can easily buy it when you arrive here.