Our billing cycle is one calendar month from the day you entered your billing details for the first time. So say…

Jan 1 you sign up for a free trial
Jan 15 it expires and you decide to sign up for a paid account, you’ll be billed that day.
Feb 15 you’ll be billed again automatically.

We’re nice people and if you sign up for a from a free trial on Jan 6, we won’t actually bill you until the 15. 🙂