It's always the same day, the next year. Pass activated October 1 2017 is good through October 1 2018.
If you activate one in May 2017 it should cover all your dates...if you activate it May 1 2017, it will be good through May 1, 2018. If you activate it May 28, 2017, it is good through May 28, 2018.
OP's question doesn't apply to this but I believe the extra month offer extends it just that month...when I renew mine that expires December 8, it will be good through January 8. Not so sure about that one though.