Is there a contradiction about the age of Earth with Quran 21:30 "...the heavens and the earth were a joined entity, then We separated them..."?

In the Holy Qur'an it says: > Have those who disbelieved not considered that the heavens and the earth were a joined entity, then We separated them, and made from water every living thing? Then will they not believe?" (Quran 21:30 ) Heaven and Earth were joined entity and after that they were separated. That means that Earth existed before the Big Bang along with Heaven (but was separated from it) but science tells us that Earth didn't exist before the universe and was created over 4 billion years ago while the universe existed long before that. Is there a contradiction about the age of Earth? Holy Qur'an says that Earth existed before big bang but science tells us otherwise. Or maybe I am just missing something?