The tax law states you have 90 days after the completion of a new home to get a mortgage and consider it home acquisition debt. Is there a definition of what constitutes "completion"?
Facts and circumstances.
See https://www.sjsu.edu/people/annette.nellen/website/225K%20Reading/N88-74.pdf
You might want to play it safe and use the first date the home is able to be occupied (or when you actually occupy the home).