Work backwards from the start date and allow time for the credential evaluation, the employer letter and any state registration. The border step itself is short; everything feeding it is not.
The short step at the end is not the one that sets the schedule
Hypothetical example: A port-of-entry TN application is decided the day it is made, which misleads people into treating the whole thing as a same-week task. The slow parts sit before it. An employer's letter may need revision before it describes duties rather than merely a title.
A credential evaluation, where one is needed, takes weeks. A state registration or licence, if the work requires one, runs on its own schedule and may require transcripts sent directly by the university. If the employer chooses to file Form I-129 instead, that adds USCIS processing time unless premium processing is purchased.
For a spring start on a farm calendar, begin in the autumn. And do not resign from a Canadian position, or commit a family to a moving date, before the permission exists. A refused admission with a resignation already tendered is a much worse position than a delayed start.