be considered contemporary if it shares stylistic, artistic, or thematic qualities with what is going on now. Scholars do remark upon writers from centuries ago who seem contemporary. Schiller wrote about alienation, composed some poetry inblank verse, and he wrote about art as "play" which became a central theme in postmodern criticism. In On the Aesthetic Education of Man Schiller talks about the potential for art to help the individual realize his human totality and for society to achieve political advancement. And this would be done by recognizing the freedom (from institutional discourses) of art as well as achieving a wholeness; Schiller saw, in his "modern" world, an increase in specialization and alienation. These two effects are common postmodern themes and they can be found as themes in subsequent works by Marx, Freud and Nietzsche. 

Artistic movements and historical eras overlap (postmodernism and the postmodern era) but there will always be times when you can find similar styles or even similar philosophical arguments in past works of art. It seems like a stretch of the term "contemporary" but in this case, it doesn't mean "from the same time." It just means there are ideological or stylistic similarities. For example, Tristram Shandy, by Laurence Sterne, is often cited as a forerunner of postmodern narrative styles. It was written in the middle of the 18th century.