Phil – It is fair to say that any instructed work after Completion Date does not instantly move Completion Date to the end of that instructed works. If you imagine a Contractor is running 100 days late, if they are then instructed one days extra work towards the end of that 100 days it would not be fair to the Employer that the right to charge Delay Damages would have been lost. In your situation, your planned Completion is beyond the Completion Date. The test will always be for any instruction what affect it has on planned Completion, and the entitlement will be to move Completion by the same date.
To take it further, if you did finish two weeks late and were charged delay damages, there is nothing to stop the Employer instructing maybe the correction of a discovered defect that is not the Contractors liability. The Contractor will be paid for the works when they do them, but will still have been charged Delay Damages for the original delay when it happened.