# The earliest time to fulfill Seudat Shelishit is from 6 and a 1/2 hours ([[Shaot Zmaniot]]) into the day. <ref>S"A 291:2, Kitzur S"A 77:16, Yalkut Yosef shabbat volume 1 page 402. The Behag quoted by the Ran [[Shabbat]] 43b "tanu rabanan" says that it could be eaten at any time of the day, but Tosfot [[Shabbat]] 118a "bimincha" and Rosh [[Shabbat]] 16:5 say that it is specifically at the start of [[Mincha]] gedola. Shulchan Aruch rules like Tosfot and the Rosh. Aruch Hashulchan 291:3 says that you don't fulfill your obligation if you eat earlier than that. </ref> If one began before the time and continued and ate at least a [[Kezayit]] after the time he does fulfill his obligation for seuda shlishit. <ref> Yalkut Yosef shabbat volume 1 page 402. Aruch Hashulchan 291:6 says also that if one began before mid-day and had in mind to continue until after mid-day and fulfill his obligation then he does fulfill it. </ref>
# The Minhag is to eat Seudat Shelishit between Mincha and Maariv/Arvit. <ref> Rama 291:2. Yalkut Yosef shabbat volume 1 page 402-403 Rabbeinu Tam quoted by the Tosfot Pesachim 105a "vihani milei", as well as the Rosh [[Shabbat]] 10:13 say that one should eat before saying [[Mincha]] because it is forbidden to drink water between [[Mincha]] and [[Mariv/Arvit|Arvit]] on shabbat because drinking between [[Mincha]] and [[Mariv/Arvit|Arvit]] could cause harm to the souls that leave after shabbat. Rambam Hilchot [[Shabbat]] 30:10 writes that it is best to eat it after saying [[Mincha]] and the hagahot maimoniot hilchot shabbat 30:20 agrees and quotes a different version of rabbeinu tam. The Rama quotes both opinions and says that the minhag is to do like the Rambam. The Magen Avraham 291:5 says this is because of the prohibition to eat before davening mincha. Aruch Hashulchan 291:4 also says the minhag is that way also. </ref> If one is unable to have it after [[Mincha]] one should have it before mincha. <ref> Aruch HaShulchan 291:4. Mishna Berura 291:11 says to make sure to eat before the time of [[Mincha]] ketana arrives. Yalkut Yosef Shabbat volume 1 page 402 says also that you fulfill your obligation but should try to get somebody to remind you to say [[Mincha]] if you are going to eat before saying it. </ref>
# Although it is forbidden to begin eating after sunset until one says [[Havdalah]], <ref> Shulchan Aruch 291:1, Rif Pesachim 21b. </ref> and it is definitely preferable to eat before sunset, <ref> Mishna Berura 299:1, Magen Avraham 299:1. See Shmirat Shabbat Kihilchita chapter 56 note 15 where he says that the idea that one should eat at least a [[Kezayit]] of bread after [[Tzet HaKochavim]] on friday night quoted in Mishna
Berurah 267:5, because there is an opinion that says you cannot fulfill a seudat shabbat during tosefet shabbat, should carry over to seudat shlishit, and therefore to satisfy this opinion one should be stringent and eat prior to sunset. </ref> some poskim give extra time to begin the meal if one hasn't yet eaten seuda shlishit. <ref> Shaar Hatziyun 299:2 gives two reasons for this. First, he says one can rely on the opinion of the Taz 299:1 who thinks that the Rosh Pesachim 12:10 allows eating until nightfall since there is a potential fulfillment of a mitzva. Second, the potential fulfillment of the mitzvah of seudah shlishit should override the potential prohibition of eating during bein hashimashot. Menuchat Ahava (v. 1, p. 184) agrees.*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. </ref>