In Britain, we are used to beginning Shabbat early in the summer so that dinner will not be too late; beginning to eat at 9.30 just doesn't work for most families. But, how do the rabbis justify declaring the start of Shabbat on Friday afternoon before sunset?
It turns out that not only do the sages permit adding to Shabbat, it is a mitzvah. Rashi (on Genesis 2:2) explains that while God knows the precise instant when one day ends and another begins, we do not. Therefore, we have to "add from the profane to the holy", meaning we begin Shabbat a little early and end it a little later (Shulchan Aruch, Orach Chaim 261:2). This is called tosefet Shabbat, adding to Shabbat.
The mandatory tosefet Shabbat ranges from 18 to 40 minutes according to different opinions. In Jerusalem we bring in Shabbat the full 40 minutes early. And then there is the option to bring in Shabbat earlier still, so the kids will be awake for dinner.