When your player is linked to your Roku account with a payment method on file Roku assigns a “bill date” to your account. Any future subscription purchases made will be set to renew on that bill date. If the date of a subscription purchase does not fall on your bill date then your purchase will be prorated to your bill date, this way all of your subscriptions renew on the same day of the month. 

For example, if you set up your Roku player on  the 15^th of the month, the 15^Th is your bill date. If you purchase a monthly subscription on the 1^st of the month you will immediately be charged a prorated amount for the period of the 1st through the 14^th, then on the 15^th when your subscription renews you will be charged the full amount of the monthly subscription. Similarly, if you purchase an annual subscription you will charged a prorated amount to your bill date and on your bill date the annual subscription will renew and you will be charged the full amount of the subscription.