Gavra-at-Work; the Mishna Berura, indeed discusses the proper time for davening Mincha and Maariv. He concludes by saying that anyone can freely choose to daven Mincha before the Sh’kia or after. Or, daven Maariv at an early Z’man (when three Medium size stars are visible.) or Rabeinu Taam. He stipulates, however, that you can’t daven late Mincha and early Maariv.
