For a port-of-entry request, the applicant can apply days before the start date once the file is complete, but should allow time to give notice afterwards. For Form I-129, allow USCIS processing time or premium processing, then notice, then the move.
Count backwards from the first day, with notice inside the count
Hypothetical example: the consulting firm wants the economist to start on the first day of the quarter. Working backwards, the provincial resignation notice period has to fit between the admission and the start date, because giving notice before the admission is the one irreversible step. For a port-of-entry request, the applicant can present the file at a land or air port shortly before the notice period would need to begin; a refused request leaves the public-service job intact.
For a Form I-129 petition, allow the current USCIS processing time, or premium processing for a fee, and only then resign. An initial TN admission may be for up to three years, and extensions can be sought while the position remains temporary; there is no fixed overall maximum, but each extension re-examines the profession and the offer. Do not list the house until the admission is in hand.
Timing review starts with the employer's genuine need date, the availability of the qualifying records, and the procedure selected. It should include time to correct a letter, obtain a licence-status confirmation, or respond to an information request. Hypothetical example: a horticulturist is expected for spring field trials, but the final job description is still being negotiated.
The employer fixes the duties first, obtains the signed support letter, and moves the start date only after the evidence is coherent. A rushed travel date can create pressure to present an incomplete description; a documented contingency plan gives both sides a safer sequence.