Where does a customer's 'join date' come from?
We use a few different things to decide the date a customer joined your store.
Firstly, we look at the customer's 'registration date', if they have registered an account on your store. If they haven't registered an account (and as such, checked out as a guest), we use the date of their first order. More about how Metorik treats guests here.
If a customer's first orders with your store were as a guest, and they later registered, we'll use the date of their first order they made with you (as a guest).