Who are you?
Now please note, I don't mean that rudely! I mean are you the Contractor or the Employer? This is a current gap that exists within the ECC contract, as the Defect Date only hangs off the Completion Date and currently ignores sectional completions. I personally have asked them to fix this in NEC4 but we will not find out next month as to whether they have indeed addressed this issue. If you are the Employer, you would have to introduce this as a Z clause I suggest to X5. This would then mean you have a sectional defect date to that sectional completion.
If you are the Contractor, then you could ask for this to be included at tender stage, but if not you would have little alternative but to price the risk of having a longer Defect Date for the section of works that is handed over earlier.