I worked it out from the counter in the my account section of the site Would be great if a dev could clear it up for us But either way I'm still happy to get my 400 cp each month
The Number Of Subscription Days listed on your account, started with the First Day After, your original 30 Day Free Period Ended with a purchase and Activation of the Game.
(assuming that you did not have any breaks with your monthly subscription till now)
I bought and installed the game on February 16th, 2010...
My Number of Active Subscription Days Counter = 606 Days as of 11/13/11.
(I had no breaks in my subscription)
That figures back to March 17th, 2010, which is 30 days after I installed and Activated the game.