Originally Posted by railways

There is plenty of detailed information here: Mobile Pass Frequently Asked Questions

You can activate the pass right up to the first day of travel. The pass is valid for one month from the the start date of your trip (which you select). So if, for example, you select July 6th as the start date, a one month pass would be valid until August 5th.

For information about making reservations with a pass, this section on the Seat 61 website is probably the best resource.