How do JCT interim payments and the due date work?

Under the interim payment provisions in section 4 of SBC/Q 2016 the cycle starts from the Interim Valuation Dates, the due date for each interim payment falls seven days after the Interim Valuation Date, and the final date for payment is fourteen days from the due date. The Interim Valuation Dates are fixed in the Contract Particulars: the first is stated and the rest follow at the same date in each month, or the nearest Business Day, running through to the end of the account, so the whole timetable can be diaried on the day the contract is signed. The Contractor may make an interim payment application to the Quantity Surveyor on or before the Interim Valuation Date setting out the sum it considers due, and the interim valuation is prepared whenever the Architect/Contract Administrator needs it to fix the amount in the Interim Certificate. The Architect/Contract Administrator issues the Interim Certificate not later than five days after the due date, stating the sum due and the basis on which it was calculated, and that certificate is the Payment Notice for the cycle. The Employer pays the certified sum by the final date for payment unless a Pay Less Notice, given not later than five days before that date, states a lower sum. Two things go wrong most often. The first is counting from the wrong event: the due date runs from the Interim Valuation Date, not from the date the application or the certificate happens to arrive, so a late certificate does not move the due date or the final date. The second is treating the standard timetable as the contract's timetable when a schedule of amendments has lengthened the period to the final date or moved the Pay Less Notice window; the executed contract, not the printed form, sets the diary.