Based on some research, I have found the following information:
Gold Challenge: Stay 12 nights within 90 days
Platinum Challenge: Stay 18 nights within 90 days

I have a few questions:
1) How can I start one of these challenges? Meaning, who do I call/email?
2) How long is the status valid until?
3) If I try for Platinum, but only do 14 nights in the 90 days, will I still get Gold?

Thanks!
--Nish