Post-modern

I'm confused, if everything up until 1945 is considered (modernism). And anything after 1945 is considered (post-modernism). So my question is, why is everything that's done today(modern day) considered as post-modern? It seems to me that (modernism) and (post-modernism) are forms of (high culture) and (post-high culture). Meaning that Van Gogh's art was considered (high culture) in his era of time. Just as equally as Andy Warhol's artwork is considered (high culture) in the present day. Both artists work reflects two diffrent ways of life in two different periods of time. Van Gogh's (peasent shoes) and Andy Worhol's (Diamond dust shoes) are the same artistic thought but in two different periods of time. Van Gogh's (peasent shoes) picture makes you wonder, why did he choose the (peasents shoes)? Did he know peasents? did he own peasents? Where as Andy Worhol's (Diamond dust shoes) picture just shows a bunch of high heel shoes rich people probably wore( no substance at all). I realize that, of course the work of two different people from to different era's of time will be distinctive. So why does it have to be labeled (modern) and (post-modern)? Who and What determines modern or post-modern?