IPS is now using the "anniversary" model for memberships, meaning that expirations are 1 year (or 2 years) from the date of joining. Previously we used the "annual" model where all memberships expired on 12/31/YYYY. As a result, most long-time memberships do still have December 31 expiration dates.
The IPS website system automatically sends out an email ahead of a member's expiration date. It also sends out a reminder 10 days after the membership has expired. It's quite possible that a number of these messages land in spam filters and are not seen.