exchanging voucher

Having done a lot of research I have just purchased 4 rail passes for my family as we are travelling to Japan in April. I understand that you have to exchange the voucher for your ticket stating which day you want to activate the tickets and reserve your seats. I also understand that you have about 3 months in which to use the rail pass. I am concerned because I have just read something that I have not come across before and am confused. From what I read it said I have to exchange the vouchers for the passes within 30 days of purchasing them? I did not see this in any other information and my voucher purchase date is about 39 days before we arrive in Japan. Please can you clarify