AT&T sets a due date. But your payment is not considered late until the day after you cycle end date.
Example: My close date is the 26th and my bill due date is the 16th of the following month. Late fees apply if it is not paid by the 26th.
In the olden days you due date was you close date. But then someone had a brain fart and came up with the new date 10 days earlier.