Do I activate my pass the same day I exchange the voucher?

Hi,
I purchased a 7-day pass for a 10-day trip. If I pick up my JR pass the day I land at Narita (10/8), does it automatically activate when I pick it up? Or does it activate when I start to use it? I don't need to use the pass until 10/10, and will need it to get back to the airport on 10/17, so I don't want to activate it on 10/8 when I pick it up.
Thanks!