Thanks for the feedback. Seems like a crock that I can't get the tIRA deduction as I was only a participant in my employers 401k plan for 1 month in 2015 and then laid off.
This "shortcut" shows up many times in the taxation world. Examples that come to mind are the child tax credit (as long as your child is born before midnight on December 31st, you get it), or being eligible for HSA contributions (usually you get the whole thing as long as you are eligible in December).
Coming up 100% fair rules that take into account everyone's messy lives, filled with divorces, layoffs, moves, children, deaths, disabilities? Not going to happen.
You got the short end of the stick this time. Expect to both "win" and "lose" a few more times over the 50+ years as a taxpayer. It might even out in the end.