EXOD1059 [T]here is a positive rabbinic mitzvah to light candles before Shabbat that will burn on Shabbat. Shulchan Aruch, Orach Chaim 263:2. On the other hand, lighting a fire on Shabbat itself is absolutely forbidden [this verse, codified in Shulchan Aruch, Orach Chaim 261:1]. Thus, lighting a fire one minute before nightfall is a mitzvah. One minute later, lighting that same fire is a sin. Thus, the sensitivity to time is needed by the Jew to know that small differences between day and night, between mitzvah and sin.