Texas Family Code - FAM § 157.266. Date of Delinquency

(a) A child support payment is delinquent for the purpose of accrual of interest if the payment is not received before the 31st day after the payment date stated in the order by:

(b) If a payment date is not stated in the order, a child support payment is delinquent if payment is not received by the registry or the obligee or entity specified in the order on the date that an amount equal to the support payable for one month becomes past due.