An existing account would be one that you already have activated once already. So, if you quit the game and unsubscribed but then wanted to try it again later, you don't get an extra 30 days when you resubscribe.
Station pass users don't get the 30 days because they're already paying one price for all their SOE subscriptions due to them all being bundled together.
Once you activate your game purchase, you'll get the 30 days after you have setup your subscription with credit card. You won't be billed until 30 days after you've started your subscription.