With Marriott's rolling qualification, they update their system ever other Monday (next one is Feb 13), so if you acquired 30 nights in 2005, and 20 nights in January 2006, the status would not update online to Gold until that specific Monday or sometimes Tuesday.

If you have already achieved your 50, and when you check-in for another stay and it shows you as Silver, just have the front desk call customer service, and it should be straightened out. It helps when you want to access the conceriege level