An open-ended description tends to hurt rather than help. TN admission is granted for a defined period supported by the employer's letter, and a letter that declines to name an end date invites the officer to ask what temporary means here.
A missing end date is a fact, not an omission
Employers often leave the end date out because they mean to be generous, and the effect is the opposite. The category is temporary by design, and the letter is where the temporariness is evidenced. When the period is blank, an officer is left to infer intent from the rest of the file, and inference is where cases get slower and thinner.
Hypothetical example: a technical publications writer near Saint-Joachim-de-Shefford receives a letter saying the assignment continues until the documentation set is complete, with no calendar attached. She is admitted for less than she expected, and the renewal arrives in the middle of a release cycle. The better letter names a period, explains what happens at the end of it, and says plainly that renewal will be considered on the same terms if the work continues.
That is not a promise of permanence and does not need to be. Two related points are worth settling early. Extension is available and is not automatic, so build the calendar backwards from the stated end date.
And a long chain of renewals does not convert the status into anything else; TN offers no direct path to permanent residence at any length.