*Mishna Berura 299:1 says that if one hasn't yet eaten the meal or is really starving then one can still eat until a half hour before tzet hakovachim. R. Avraham Chaim Naeh in Ketzot HaShulchan (chapter 94 note 6), notes that Mishna Berurah's ruling that one may eat until a half hour prior to nightfall is following the opinion of Rabbeinu Tam that nightfall is seventy-two minutes after sundown. However, according to the Vilna Gaon, nightfall can be within one half hour of sunset. If so, one who follows the opinion of the Vilna Gaon would be prohibited from starting to eat one half hour before nightfall (of the Vilna Gaon) which is prior to sunset. Rav Shlomo Zalman Auerbach quoted in Shemirat Shabbat Kehilchata chapter 56 note 17 says that since even those who follow the opinion of the Vilna Gaon with regards to when to end shabbat will never actually recite Havdalah within one-half hour of sunset, they may start eating up until sundown.
*Rav Moshe Feinstein quoted in The Radiance of Shabbos Chapter 15 note 36 can start up until nine minutes after sunset. Pninei Halacha [[Shabbat]] vol. 1 page 130 quotes Rav Moshe Feinstein Iggerot Moshe 4:69:6 that one has until thirteen minutes and a half minutes after sunset.
*Yalkut Yosef shabbat 1 page 414 allows one to start until thirteen and a half minutes after sunset.
#If one began eating before sunset he may continue after. <ref> Shulchan Aruch 299:1, Aruch Hashulchan 299:3. Mishna Berura 291:2 says that having said the beracha is enough of a start to your meal to allow one to continue afterwards. He also quotes an opinion that this only applies during bein hashemashot but after tzet hakochavim one would not be allowed to continue. Rama says that the minhag is not so, rather one can continue even after tzet hakochavim, and the aruch hashulchan 299:4 says that is the minhag. </ref> However, this doesn't apply if one was only drinking not as part of a meal <ref> Shulchan Aruch 299:1 </ref> or to eating only mezonot or fruit, so one must stop if that is all that he is having. <ref> Aruch Hashulchan 299:5 </ref>