It is a requirement under the NEC3 Engineering and Construction Short Contract (ECSC) that completion is when all works have been completed in accordance with the works information and are free from any defects which would prevent the employer from using the works and others from doing their work.
Where it is the case that there are defects present on the original completion date that would prevent the employer from using the works and these defects are subsequently corrected a few weeks after the original completion date, which date should be stated on the completion certificate: the original completion date or the date upon which the defects were deemed as having been corrected?


We think you may be confusing two different defined terms, that is completion and the completion date. Completion is about the state the works are in, defined by whether or not they comply with ECSC clause 11.2(1). If they do then completion has been achieved; if they do not then completion has not been achieved. The employer is required to certify the date when completion is achieved, see ECSC clause 30.3.
The completion date is the contractual date by which the contactor is obliged to achieve completion, see clause 30.1. It I the completion date set out in the contract data unless that date has been changed in accordance with the contract, see clause 11.2(2). Usually the only way that the completion date can be changed is because of a compensation event, see clause 63.4. If the contractor fails to achieve completion by the completion date, then it is in breach of clause 30.1 and it has to pay the delay damages set out in the contract data (see clause 50.4), which are deducted as part of the usual payments due, see the third bullet of clause 50.3.
So, in your case, the employer should certify completion as the date when completion, as defined by the contract, was achieved. The completion date is irrelevant in this process.