It is a safek when night is so erev shabbos we are machmir like the Yereim to stop work 18 ninutes before the first shkia and motzei shabbos doing work by the second shkia when it is completely dark. Since davening is rabbinical, we daven maariv 12 minutes before. The Baal Hatanyeh changed his view.
In the SA he paskens like the Rabbenu Taam whereas in his Siddur like the geonim where there is only one shkiah and 13 1/2 minutes is ben hashmoshos. Those who go by the Rabbenu Taam can daven Mincha until 58 1/2 minutes before his shkiah and then Maariv.
