Update the actual assignment and assess the temporary-entry evidence rather than relying on an obsolete end date.
Use the current duration and duties
A commercial extension does not itself establish immigration permission for the revised period. Document the real terms and appropriate process. Do not preserve a shorter date in the file merely because it appeared in the first proposal.
Seasonal work in process industries generates pressure to have paperwork ready months in advance, and that pressure encourages applications built on an outline rather than an offer. If authorization is needed, its route and timing depend on the actual facts; do not promise the organization a start date until those steps and the effective permission are clear. If the file is not ready, returning to Canada and completing preparation there is a legitimate outcome rather than a setback, particularly for a Canadian citizen who may later seek TN admission at a port of entry. Turn the target start date into a sequence of dependencies: settled duties, completed employer letter, credential availability, procedure choice, and travel date.