In contracts with a single payment, the payment term begins on the date of receipt (or verification) of the goods or services, so normally the main obligation of the PO is met before the payment deadline began.
However, additional obligations may be established beyond the payment term. In this regard, it must be noted that the Law establishes that late payment interest will accrue when a party has fulfilled its contractual and legal obligations under the contract. Nevertheless, it is reasonable to assume that this only refers to the main obligations under a contract, and does not apply to additional obligations set forth after the payment term or to obligations surviving the termination of the contract e.g. confidentiality provisions, guarantees or liability obligations).