I assume you've made gold by staying 50 nights so far in 2010. On that basis if you do another 25 nights this year you will attain Plat, though only till the same end date as your gold, Feb 2012.

Marriott has an unwritten policy that it doesnt undertake double downgrades, so PLT Prems are downgraded to PLT's, PLT's to Gold, Gold to silver, no matter how few nights acheived during the year immediately preceding the downgrade. Hence in your case if you acheived PLT this year but stayed less than 50 nights (or indeed not at all) in 2011, you'd only be busted down to Gold for period ending Feb 2013.

It is worth remembering this is an unwritten rule and such understandings can go south pretty rapidly.

Who remembers the rolling calendar?