Looks like its any time up to the first day of work, however the wording is a little confusing. How can you start work without accepting a job offer?
>> Employees must complete and sign Section 1 of Form I-9 no later than the first day of employment, but not before accepting a job offer.
> I'm pretty sharp with US employment, tax and passport forms, yet that's the first time that I've heard somebody being banned for an I-9 mistake.
The form clearly states some severe penalties for not answering honestly. Seems like the author got off lucky!
>> I am aware that federal law provides for imprisonment and/or fines for false statements or use of false documents in connection with the completion of this form.
>> I attest, under penalty of perjury, that I am (check one of the following boxes):
