NEC3: Option A.
For issuing the final account on projects: CL.50.1 requires the PM to make assessments of the amount due up to 4 weeks after issuing the Defects Certificate. The Defects Certificate is a fixed time period from Completion, often 52 weeks. Is it fair to state that the correct final account date would be “four weeks after issuing the Defects Certificate” (completion+52weeks+4weeks)? This would take account any charges the PM/Employer incurred for correcting any uncorrected defects by the defects date (i.e. the original Contractor did not correct their own Defect).
I’m glad you put final account in some “” because the actual answer is that there is no such thing. There is a final assessment and therefore payment, but you’ll notice the lack of capitals, because that’s not a defined thing, its just the last time any money changes hands.
Turning to the question, we have to think about the defects process. You can notify defects up to the defects date. You issue the Defects Certificate on the later of the Defects Date and the end of the last defect correction period. So your formula above is only correct if there are no outstanding defects at the defects date. Otherwise it is (end of last defect correction period + 4 weeks).