activating JR pass

we are headed to Japan in late June for 12 days but are going to purchase the 7 day JR pass. Since we are buying the pass specifically for the Shinkasen train to Osaka, do we just activate/use the pass starting on the day we get on the bullet train to Osaka? or 7 days before the bullet train trip heading back from Osaka to Tokyo? are the dates flexible for the 7 day pass? thanks