As soon as you start a subscription your billing period starts as well. This can be any day in a month. 

The monthly billing period lasts 30 days and the yearly billing period 365 days.

We will charge your payment method on the first day of your billing period.

The subscription will renew itself automatically if you don’t cancel two days before the end of your billing period.

You can cancel any time. The subscription will vanish from your account automatically at the end of your billing period.