I called Marriott today (April 6) and talked to a very helpful agent who enrolled me in the Platinum Challenge in under 5 minutes. I am currently a Silver member, and the agent told me that I have 3 months to complete 16 nights to attain Platinum status. (The Gold Challenge of 8 nights in 3 months is also still available). Interestingly, she specified that I have until July 31 to complete the challenge. I asked her to verify the challenge end date, and she once again confirmed July 31. This would imply one actually has 3 calendar months to complete the challenge. I realize this is a different timeline than other posters experienced. I plan on reaching 16 nights well before July anyway, but I wanted to take a moment to provide a recent data point for those interested in a Marriott status challenge.