Making Early Shabbat

There is a mitzvah to add on to Shabbos. What is the halachic status of the time added onto Shabbos? Does taking Shabbos early have the status of a neder?

When is the earliest one may accept Shabbos? Can one daven Maariv early?

Delivered at the OU Israel Center, July 4th, 2018, 21 Tammuz 5778

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