The answer is yes.

According to some of the Marriott insiders on this forum and to posts from members here, Marriott will use a rolling 12-month window for your upgrading you to the next higher elite level. Once you attain that elite level, Marriott will stick with the calender year accumulation.

I would suspect though, that if one fell to a lower level, and then was able to accumulate the needed night credits within a rolling 12-month window, one would again be upgraded.