Many crossing currently INSIST that you are beginning your TN job on that crossing, and will refuse to give you a TN until you are about to start. This was not the case in the past, but is becoming more and more so. Used to be you could go get your TN, then go home and pack, quit your Cdn job... and then go down, secure in the knowledge that you got your TN (you would 'lose' 2 or 3 weeks of the TN). Now you pretty much are expected to take a jump into TN with both feet -- again, this is at most POE/PFIs.
Your TN letter should be dated, but there should not be any start date in the body, just that you will work for a period 'not to exceed one year'. The TN will be granted for one year from the processing date, regardless.
What most do these days, is to arrange a 'meeting' with their new employer (even by phone) so that they can truthfully say that they are going directly to work after they get the TN. This may be difficult to believe if you are at a land crossing with no gear in your car, and your job is in, say, Texas.
TNs are granted assuming you will start that day. It doesn't matter when you actually start, so long as you can convince the officer that you aren't just getting the TN and turning back to Canada.
