How does NEC4 clause 51 work?

Clause 51 turns the clause 50 assessment into a certificate and a payment, and every period in it runs from the assessment date, not from the certificate. Under clause 51.1 the Project Manager certifies a payment within one week of each assessment date. The first payment is the amount due; each later certified payment is the change in the amount due since the last certificate, so a certificate can be for a payment from the Contractor to the Client where the amount due has fallen. Under clause 51.2 each certified payment is made within three weeks of the assessment date, or within a different period if one is stated in the Contract Data, and if a payment is late interest is paid on the late amount from the date it should have been made until the date it is. Under clause 51.3, where an amount due is corrected in a later certificate, whether because of a mistake, a compensation event or a decision in dispute resolution, interest is paid on the correcting amount from the date the incorrect amount was certified until the date the correction is certified. The interest rate and its calculation are in the contract and are not repeated here. Where the contract includes Option Y(UK)2 the due date, the final date for payment and the payment notices under the Construction Act sit on top of this clause, and that page explains them. What goes wrong: Contractors and Clients count the three weeks from the certificate, which shortens or lengthens the period by however late the certificate was; a certificate that arrives late is treated as postponing payment, when the payment date has not moved; corrections are certified without the interest clause 51.3 attaches to them; and Contract Data periods that differ from three weeks are missed because nobody read the entry. Read the certificate against the assessment date every time.