Your recurring billing date is determined by your original sign up date. If you placed your first Autoship order on the 15th, your next shipment will be billed and processed on the 15th of the following month, or the month after if you elected for bi-monthly shipping, and so on.
If you pause your Autoshipment, your recurring billing date is determined by the date you resume your account. For example, if you have an order process on 9/22 set to monthly billing, then pause your account on 9/23. If you reactivate on 10/5, your next shipment date will be 10/22 since an entire month hasn't passed. However, if you reactivate on 10/30, your order will automatically process and your monthly bill date will reset to the 30th of each month.
Further, if you utilize the SHIP NOW feature, your recurring billing date will reset based on that date. For example, if you have an order process on 9/22 set to monthly billing, then use the SHIP NOW feature on 10/1, your next shipment date will be 11/1 or a month since your last shipment.