This is very similar to InterContinentals' system for status. You need 30 night in a calendar year to reach Executive status. As soon as you hit 30 nights, your year starts over. If you then hit 50 nights in the next 12 months, you recieve Ambassador status, and a new year starts.

However, given how restrictive Marco Polo is, maybe you should consider joining one of the other OneWorld programs. The only downside is slighly less chance for an operational upgrade...