TN-1 job offer doesn't have end date


I realize that TN status can be renewed indefinitely, but does the TN Support letter needs to have an end date? What if the job is "permanent" and does not have an end date? Will an end date still needs to be added to the Support Letter?
Thanks nelsona. I have just received my TN support letter from my prospective employer. It states that there is no end date for this job, and now I'm a bit worried. I do have strong proof of ties in Canada, like family, property, and various investments (TFSA, RRSP, mutual funds, etc) that I am not cashing out.
By the way, if the job itself is "permanent" with no end date (and the offer letter to me doesn't state an end date), can the TN employment letter state an end date that is 3 years from the start date even if that is not in the job offer letter?