Is the 7-Day pass locked to a week or can I choose the day it starts?

Me and my girlfriend have tickets to and from Tokyo for our 14 day stay. We plan on getting a pass for 7 days to have a brief stay in Osaka and visit other areas in South Japan during the middle of those 14 days. Essentially Tokyo > Osaka > Tokyo.

I'm wondering what times the passes are activated to make sure we aren't stuck having to pay a ticket price for the Shinkansen in Osaka to go back to Tokyo. We want to maximize our days in Osaka, but at the same time, not feel rushed for our last few days in Tokyo.

Is the pass active for the 7 days from the point you activate it? Is there a specific time they activate and deactivate? Or is it all set differently?

Any help is appreciated!

Thanks