Lautenberg says that ANY misdemeanor "domestic violence" conviction applies, no matter how old. Suppose you were convicted of misdemeaor assault of your now ex-spouse 18 years before enactment of Lautenberg. At that time, a lawyer would have explained your options to you and the "liabilities" for each plea (guilty, not guilty, no-contest, etc.). He may have advised you to plead guilty because at that time the liabilities would be a fine, "X" days in jail, possibly both and a minor criminal record that would have minimal impact on your future.
Thus, you plead "no contest" and were "convicted". Then, 18 years later you find you can not own a firearm. Huh? The Lautenberg law, in effect, changes the impact of your plea to a prior charge.