Do not assume a material unresolved employment fact can be ignored because work is scheduled.
Use the actual confirmed proposal
Review the employer and duties through the appropriate process before relying on authorization. A start date does not decide the entity question.
Processing times vary by authority and season, so start the renewal while the employer letter and credential documents are still being prepared. If the season is likely to arrive first, ask the employer whether the program can begin with other staff, or whether desk-based preparation can proceed from Canada while the file is completed. Set your own checkpoint several weeks before the intended start: if the employer letter and the professional's description are not in hand, the start date should be discussed rather than assumed. Canadian citizens may request TN classification at a port of entry or use an employer-filed Form I-129; neither option lets the project schedule confer work authorization. The period granted relates to the employment described in the employer's letter, and the officer decides the admission period; it is not set by the applicant's preferred schedule.