I've traveled Amtrak quite a bit over the years, but I've never used a USA Rail Pass until now. I purchased mine on January 18, and the receipt says it is valid until May 17. Does this mean that I need to book my trip using the pass before May 17, and travel anytime thereafter, or does it mean that I need to end my trip by May 17? I can do either, but I really want to travel the last part of June. Thanks in advance for your guidance.