I have the same question. I found this on their website which would lead one to believe that it's a calendar year requirement ...

Elite Membership Requirements

1. Only nights personally stayed and individually billed at participating hotels are credited toward Elite membership and renewal. Accounts may not be combined to achieve Elite membership.

2. To achieve or renew Elite status, the following number of personal nights must be stayed between January 1 through December 31 of any given year at participating locations:

Silver 10 to 49 nights
Gold 50 to 74 nights
Platinum 75 or more nights