Allow time for the credential review and the employer letter, then decide whether a same-day port-of-entry application or a petition with a written decision suits the risk. Neither approach should be booked against a fixed site-mobilization date until the file is complete.
Work backwards from the day the contract needs you on site
Inspection and construction work in the northeastern United States is seasonal, and an employer's mobilization date is usually inflexible. Hypothetical example: a Boischatel engineering firm's Vermont client wants inspectors on site by the first week of May. Counting back, the employer's letter must be final and signed, the credential originals in hand and translated, and the applicant's citizenship and immigration history reviewed, all before travel is booked.
If the file has a complication, a prior refusal, an unusual credential, or an ownership stake, a petition filed by the employer gives a written answer before anyone drives to the border, and premium processing can shorten that wait for a fee. Admission at a port of entry is quicker but leaves no room to correct a problem that day. TN is granted for up to three years at a time; a contract longer than that will need an extension or a fresh admission, which should be diarized now.