Just a few additions to what has been said. Your nights toward the renewal of your platinum status will be counted until December 31st. Your status continues until the end of February in order to get the new card out. Once an account has been downgraded it will be looked at every Monday when the system checks to see how many nights have been posted to each account in the last 12 months.
Several post have mentioned that accounts are not always down graded even when there have not been enough nights during the calendar year. This is most likely because of something we call tenure renew. Basically we look at the amount of stays a member has had over the last three years and renew the member’s status based on thresholds that show a pattern of long term loyalty.