No. A signed offer describes intended employment; it does not by itself authorize U.S. professional services. The timing of entry, commencement and any material change should be planned around the applicable authorization.
Put the start date after the permission question
Hypothetical example: a landscape architect signs an offer for a March start but the employer changes the worksite and adds a management function in February. Rather than treating the original letter as permanent permission, compare the revised facts with the TN occupation and supporting record. A Canadian may present a supported TN request to CBP when seeking entry, or an employer may use Form I-129 where appropriate.
Neither option permits starting early simply because equipment must be installed or training is scheduled. Identify the intended border crossing or petition timeline, the first day of U.S. activity, the expiration date sought, and the plan for any continuing Canadian work.
Leave sufficient time to correct credential, licensing, or duty-description problems. If the person is already in the United States in another status, do not presume that arrival as a visitor converts into authorization for the new role. Timing decisions should follow the actual classification and approval facts.
Use a written calendar for petition validity, status expiry, school dates, travel, planned work and any payment deadline. If a new fact moves one date, reconsider the sequence instead of treating the original plan as fixed. Timing evidence should support, not outrun, the authorization sought.