Work backward from the credential, since it is the slowest item. Get the employer letter drafted only once the credential and citizenship documents are in hand, then choose the port of entry date with a margin for questions.
Let the slowest document set the start
A quarter-start date is a business preference, not a rule that shortens anything. The items that take the longest are a foreign-credential evaluation, a replacement degree certificate from a university and, where needed, a corrected employer letter. Order those first. The letter should be finalized last, because it must describe the credential and dates that the documents support. Once everything is assembled, a Canadian citizen can appear at a port of entry with little notice, but plan the crossing for a day when a refusal would not strand the household. If the employer prefers to file Form I-129 in advance, allow for USCIS processing and consider whether premium processing is offered for that form. Tell the plant that the start date will be confirmed when the admission is confirmed, not before.
Hypothetical example: a Canadian college teacher is asked to begin a semester in six weeks. The school should first issue the detailed offer and the teacher should collect citizenship and credential proof, then choose between requesting TN admission when ready or having the employer pursue a petition. The timetable is settled by the document that takes longest to obtain, not by the class schedule. If a degree verification or state authorization is still pending, the start date should move. Preparing lectures or teaching from within the United States before admission risks turning a scheduling problem into an immigration problem.