* S"A 261:2 writes that one should accept [[shabbat]] early during the time between [[Shekiyah]] (sunset) and [[Bein HaShemashot]] (twilight) to fulfill the mitzvah of Tofeset [[Shabbat]]. S"A explains that this time period is the length of time it takes to walk 3.25 [[mil]]. Then [[Bein HaShemashot]] (twilight) lasts for a period of 3/4 of a [[mil]] which is followed by [[Tzet HaKochavim]] (emergence of the stars). For this discussion, we're assuming that a [[mil]] is considered 18 minutes like the ruling of S"A 459:2 and Mishna Brurah 459:15. If so, S"A holds that one must accept [[Shabbat]] 13.5 minutes before [[Tzet HaKochavim]] which is 58.5 minutes after [[Shekiyah]].
* However, Mishna Brurah 261:23 quotes many Rishonim and the Gra who hold that between Sheiyah and [[Tzet HaKochavim]] there is a short [[Bein HaShemashot]] of 3/4 of a [[mil]] (13.5 minutes) and afterwards it's considered nighttime Deoraitta. According this opinion the Mishna Brurah writes that one may absolutely stop doing Melacha (activities forbidden on [[Shabbat]]) before [[Shekiyah]].
* The 39 Melachos (Rabbi Ribiat, vol 1, pg 145) writes that the accepted practice is in accordance with the Gra and so it's strictly forbidden from doing Melacha from the time of [[Shekiyah]]. Yalkut Yosef ([[Shabbat]], vol 1, pg 128) agrees. </ref>