7th of Shebat, 5770
Officially, Shabbat starts at sunset (Shekia). Sunset or sundown is when the sun disappears from the horizon. It wasn't easy to know when does sunset time precisely occurs: it changes virtually every day and from location to location, even within the same State. Besides, if it is a cloudy day it could only be estimated. Nowadays, BH, we can know the exact time of sunset checking in any newspaper or online http://www.ou.org/holidays/calendar#calendar or calling 5167962626 (My Zmanin).
Although Shabbat officially begins at sunset, we receiveShabbat earlier than sunset time. Why? Because we have a Mitzva to add time to Shabbat, before it starts and after it finishes. So we take a few minutes before sunset and “convert them” into Shabbat time and we do the same at the end of Shabbat, extending Shabbat a few minutes after we see the 3 stars.
How long before sunset we should receive Shabbat? The conventional custom is to receive Shabbat 18 minutes before sunset. In case of need, however, one can still light the candles or do anything up to five minutes before sunset. The Mitzva of adding time does not specify how much time to add, as long as we add “some” time before sunset.
In most calendars, nowadays, we are given already the time of candle lighting time, which is 18 minutes before sunset. Today, Friday February 22 , 7th of Shebat, for example, sunset is at 4:59 and Candle Lighting 4:41.