Can I change the days on which I wish to use the Japan Rail Pass?

Once your Japan Rail Pass has been activated, you can no longer change the selected days

When you swap your Exchange Order for the actual Japan Rail Pass you are asked to select the day you want to start using the pass. This day is referred to as the ‘activation date’. If you select the April 1 to be your activation date for a 14-day pass, it will be valid until April 14 (until midnight).