To summarize (given some mis-information above):

Stay the two remaining nights during the challenge period and you will have Platinum status through February 28, 2014. You do not need any more nights this year or next to achieve that.

To renew Platinum status from 2/28/14-2/28/15 you would need to stay 75 nights between 1/1/13 and 12/31/13. 10 or 50 nights in the same period would earn you Silver or Gold status, respectively.