If a search warrant is dated 2 days after a search, is it still legal?

This would be grounds to challenge the validity of the search warrant. Typically, a search warrant must be found to have probable cause for the search via a Judge or Magistrate prior to its execution. A search warrant that was not signed off on prior to the search is invalid unless the prosecutors can show that exigent circumstances nullified the need for the warrant at the time of the search.