I actually don't agree with using a cut-off date to designate what's a classic and what isn't. Besides the obvious problems with the fuzziness of dates, I also just think that there is more to it than that. There are works from before WWI which are still read on occasion but are probably not considered classics; for example, I recently read a poem called "To His Royall Highness," which was written during the Renaissance but which I doubt if anyone would consider a classic. In fact, I only read it because it had value as a historical artefact, rather than as a piece of literature. That, I think, is an important part of what makes a classic a classic: it has to have "literary value." That takes time to get established, of course, which means that it's odd to talk about a contemporary classic, but I don't think that age is the only factor. To use the language of logic, age is necessary but not sufficient to define what a classic is.
I'm sorry to sound cynical, but I think a classic is really just what people decide it to be. Certain books get labelled that way, and as times change, other books get added to the list. The canon in the past used to be a lot more restrictive than it is now, but I still think that it is surprisingly arbitrary even now. When you think about the definition of classics as those books which have "withstood the test of time," it makes me wonder why it is that some books have while others haven't. The cynic in me wants to suggest that sometimes it is more a factor of pedagogy, elitist attitudes, and what have you, rather than an actual "universal" quality of the writing itself.
Sorry if this is an unwelcome opinion... I also realize that I could have maybe phrased things a bit nicer too. Oh well!