Historically, “Eastern Europe” has been used as more of a derogatory signifier in places where it’s culturally acceptable to make jokes at the expense of Slavs, like in Germany or USA.
Let’s not forget that Vienna is actually geographically further East than Prague, though I doubt many Austrians would be comfortable considering themselves Eastern European.
Poland is pretty squarely in the middle of Europe by a number of metrics.
If we’re talking about time zone, Poland uses Central European [Summer] Time.
If we’re talking geographically, most of the claimants to the geographical midpoint of Europe are actually East of Poland.
If we’re talking politically, then the midpoint of the European Union is said to be in Würzburg, which is only 470km South-West from the border with Poland. And we can hardly say that anything East of Würzburg is Eastern Europe, since that would include all of Austria.