JR pass and N'EX

Hi,
If you choose to use your JR Pass to take the Narita Express, then you have activated it and the clock is running.
Please remember also that the day you activate it for counts as Day One - which starts and ends at midnight. So if you activate it and use at 6 PM, your first "day" is just 6 hours long.
So if you want to start using your JR Pass from another date, that is fine. You can pay the fare for the Narita Express (very pricey), or use an alternative like a [Keisei/Metro] package or a cheap 1000 yen bus.