Humor Obsolescence

What happens to humor that missed it's era? There has always been "timely" and topical humor that has a limited shelf life, or humor that requires the audience to have specific knowledge, but what happens to humor (and the energy required to generate it), that is created way past the time it has a connection with any audience?  Can it even be considered "humor", or the very fact it exists outside the realm of conventional humor humorous in itself?