I'm certain that musicologists would have a great time discussing this term. I had a text for a music appreciation class in 1966 that more or less used the term 'contemporary' to mean 20th Century, even late 19th Century. That clearly isn't working here.
In non-technical terms, I think that modern and contemporary could be used as synonyms. You might check with the staff at the CD store, although I realize that many of them may not be 'musicologists'.
When we say so-and-so A was a 'contemporary' of so-and-so B, we of course mean that at least parts of their lives overlapped. Perhaps, as far as composition is concerned, we look at the immediate influence of Berg's music on what is happening today to determine if it still should be called 'contemporary', or even 'modern'.