[RIP Robin Williams. You were a versatile actor, indeed].
The piece of news from Huffington Post reads...
In 2006, after 20 years sober, he checked himself into rehab for alcoholism. He opened up about his struggles with addiction to alcohol and cocaine in a powerful interview with The Guardian and on "Good Morning America."
What sort of use is that? An adjective after a noun clause without any preposition? Or better use noun for that reason? ...after 20 years of soberness... or something the like?
I checked all the examples of the word sober here, on MW but none of them fits the structure of HP.