The employer sets the start date first and prepares the letter last. A port-of-entry application can be decided in one visit once the letter and degree documents are complete, so the delay is nearly always in producing a correct letter, not at the border.
Work backward from the letter, not from the field season
Agricultural employers plan around planting and harvest, so the offer usually comes with a fixed start. The sequence that fails is: accept the date, give notice, arrange housing, then discover at the border that the letter is defective. Reverse it.
First obtain the letter and have it reviewed; a rewrite typically takes the employer a week or two, especially if a human-resources department must approve wording about duties and compensation. Then assemble the degree, transcript and passport, and choose the port of entry or preclearance location. Apply with enough margin that a refusal can be corrected and re-presented before the season starts.
Only after admission should the applicant resign and move. If the employer needs help before then, remote preparation from Canada, such as reviewing trial protocols or grower data, does not require US work authorization. TN can be granted for up to three years and renewed, so there is no advantage in rushing the first admission to fit a single season.