Modern is an adjective meaning "relating to the present-day or to the most recently developed technology, architecture, medicine, art, etc".
When some people assert that the early 20th-Century is the Modern Era, and the latter part of that century is the Post-Modern, they are attempting to destroy the whole sliding-scale aspect of the above meaning.
If you value the meaning of "modern" then please resist this modern trend toward the use of "post-modern".