I had a Craftsman ft/pound torque wrench for over a decade assuming it to be calibrated from new. I later bought two more, one in inch-pounds but never had the need for either.
Then about six years ago I took the two new unused ones into the missile assembly building and tested them on their torque testers- both were so far off I'm glad I never used them. Went to Sears and they were useless- torque accuracy is not part of the unlimited warranty, even from new.
So a few days later I took the big one I had been using and tested it, and it was off by 10# low. So the score at this point is three Craftsman torque wrenches that I can't use for anything more critical than a wheel torque.
Last year I bought a HMMWV and the hub torques are very important (well, they are if you want the wheels to stay on) so I asked around to find a place that would calibrate them all. It was cheaper to buy new ones, at the same time recognizing that the new ones might not be any more accurate than the old.
I found several videos describing how to use weights and calibrate them at home, and I did all three. Last month I used a Harbor Freight coupon to pick up a ½" torque converter, and used that to test the two ft/pound wrenches and they came to within 1# of each other.