Your billing date is the day you first sign up for your account. You receive a recurring charge for your membership on this day each month or on this day each year annually. Once the account is active there is not a way to change the billing date.
You can view your billing date by clicking on the menu icon at the top right corner of the control panel and selecting ACCOUNT and then MEMBERSHIP.
Listed here you can see your total charge as well as your next billing date.