Rabbi Moshe Feinstein describes at length in Igrot Moshe Orach Chaim 4, 62, how one should act regarding times in America. How does Rav Moshe Feinstein hold in his Igrot Moshe Orach Chaim 4, 62, when it comes to the proper times of Kriat Shema Shel Aravit in America? If I Daven during twilight do I have to repeat the Shema later in the night according to Rav Moshe? (Tzet HaKochavim is about 8:00 PM, My shul davens Aravit at around 7:35-7:40. Do I have to repeat Shema later on in the night?)
Rabbi Moshe Feinstein ZT”L discusses a wide range of topics that have to do with nightfall in Orach Chaim 4, 62, such as Motzaei Shabbat, end of fasts and more. In Igrot Moshe Even Haezer 4, 84. he writes that the time of Kriat Shema Shel Arvit in New York is about three quarters of an hour after sunset. For more exact times according to your exact location see: http://www.myzmanim.com/default.aspx In your case all agree that you definitely have to repeat Shema after nightfall. It is stated in Shulchan Aruch Orach Chaim 235, 1. that if one says Kriat Shemat before nightfall he is required to say it again after nightfall without repeating its Brachot. The Mishnah Berurah 6 adds that it is not a stringency but Me’ikar Hadin (the actual law).