Treat the proposed date as a planning target while the underlying eligibility, evidence, and admission process are actually reviewed. Preparation and the government's decision are separate steps, and an expected date is not a form of authorization to work.
Identify what still needs to be confirmed
Ask the employer what remains unresolved in the offer and who is responsible for confirming it. Check current port-of-entry or petition procedures before booking non-refundable travel. If the employer, the assigned duties, or the client changes before the applicant travels, treat that as a reason to reassess rather than proceeding on the original paperwork. A temporary TN admission does not remove the need to consider the applicant's broader immigration history and intent.
TN is granted for a period not exceeding three years and can be extended or a new admission sought, but each renewal is decided on the duties as they then stand, which on a long construction project are rarely the duties described at the outset. The immigration step is usually the shorter one; an admission in TN status can be sought once the letter and credentials are ready, and admission is granted for a period set by the officer, with extensions possible while the employment continues.