Once the corrected letter is in hand, she may seek TN admission no more than 10 days before the employment validity period begins, although she may not work before that period starts. The practical schedule is set by how long the employer takes to produce a compliant letter and by the procedures of the chosen CBP location.
The employer’s letter, not the border, sets the timeline
A Canadian TN applicant may seek admission at a CBP-designated port of entry or designated preclearance or pre-flight inspection station no more than 10 days before the employment validity period begins; approval is decided during inspection and is not guaranteed. She should confirm the chosen location’s current procedures before travel. Hypothetical example: the immediate constraint is upstream: the human-resources manager must obtain a rewritten letter, have it reviewed and signed by an officer of the firm, and send it before the trip.
If the employer prefers the USCIS petition route, regular processing has no guaranteed timeline and premium processing uses the current published window, so the start date should follow the expected approval. Extensions can be sought by a new application for admission or by petition before the current period expires.