When can I enter US under TN Status?

We have had many TN/TDs and all but one expired one year less a day from the day we crossed the border, however, the last one was dated to expire one year plus 3 days later using the date on the TN request letter (we don't use an actual offer letter but a special TN request letter drawn up with all the necessary components). I asked about the extra time and the officer said that they could include some travel time at their discretion.

They usually tell you you have to start working once you enter the U.S.