Given the safety and build of these tanks, if you still have CO2 in the tank by that date, I would say that you should be fine to use it up, even after that date. I would definitely recommend a visual examination to make sure there isn't corrosion or anything else worrisome. If there is anything you would be worried about though, definitely get it recertified.
To give some context to my belief, I have two 50# tanks. I used to keep one for serving duty, but when only using for serving, the full tank lasted over 9 years, and I never felt it was dangerous during that time. This was a steel tank that had been manufactured in the 40's too, and still looked great.
I have five 20# tanks along with the two 50# tanks, and with that many, some go out of test. I always use up the tank before swapping it.