Generally under the NEC3 Main Options C,D,E.
Clause 11.2.(29), focusing specifically on ‘before the next assessment date plus the fee.’
If the current date is 15/07/2019 and the assessment date as 31/07/2019. The Subcontractor interprets the next assessment date as 31/08/2019. Where as the Contractor interprets the next assessment date as 31/07/2019.
Which is correct?
I am assuming you are referring to the ECS form rather than ECC as you mention Contractor and Subcontractor, although the principle is the same for both.
Under NEC3 the Contractor assesses the amount due at the payment assessment date. Under Main Options C, D and E the definition of PWDD includes a forecast of what will have been paid by the Subcontractor before the next assessment date.
If the current date is 15/07/2019 then the next payment assessment is made on 31/07/2019 and would includes a forecast of what the Subcontractor will have paid by 31/08/2019.