Allow time for the person with authority over the role, not just HR, to confirm duties, duration, and pay, and for counsel to review the result. Build that into the start-date discussion rather than requesting the letter at the last moment.
Sequence the letter ahead of the start date
Employers often need internal sign-off before committing to a stated period or a duty description, and that can involve a hiring manager, a legal contact, and a finance approval. Ask early who must approve the wording. If the proposed start date leaves no room for that, say so and discuss moving it, since presenting an inaccurate letter to meet a date is not a workable trade.
At the border, renewal can be done any time before expiry with a new letter, ideally weeks ahead. Regulator confirmations, credential evaluations and corrected employer letters each run on someone else’s schedule. The state regulator sets the true earliest start date, and the offer letter should be dated to match it. Once the credential is settled, a Canadian citizen may apply at a port of entry, or the employer may file Form I-129; no particular processing time or outcome is guaranteed.