Global Pass - should I get the 3 month or 2 month pass?

Hey all, 

I am hoping to get a pass but I am a little confused on which to purchase. 

I will be traveling in Europe Between end of September and beginning of November. 

Does this require me to get the 3 month pass or the 2 month pass? 

Does the pass work from (example) August 12 - October 12; as a two month plan? 

Or the is the 2 months considered to be August and September only ?