My wife and I will be going to the Bernese Oberland area staying in Wengen for 10 days. I plan to get the Half Fare pass as well as the Bernese Oberland pass. Is it possible to buy these ahead of time? I understand the pass timeframe starts on their first usage, so I was wondering whether using the Half Fare pass to buy the B-O pass will "trigger" the first usage date? In other words, if I buy the passes in mid-July, would the Half Fare pass expire in mid-August or would its timeframe start in September when we arrive and use it there?
Also, is there any advantage to buying the passes early or is waiting until a week before departure ok?