It's pretty simple, they show "cleared balance" on your "balance of account" page and once that is above the threshold, the "balance not due" line underneath it turns into "Next payment date".
At least, that's how it should work, but having just got this month's payment I am now showing $75 cleared and "payment not due". I assume that is just because the payment change was recorded today and that tomorrow's update will go to Payment Due Jan 1. Or maybe it has to be $75.01 to qualify, rather than $75.00.