Well, עֶ֫רֶב evening is only the time from dusk onwards, including the first hours of the night. PS. Note that in religious contexts or before temporal expressions, it even means only the time before it or the eve before it, like עֶ֫רֶב־שַׁבָּת is Friday evening until the sun is setting.
Well, I thought it to be a cognate of Arabic غَرَبَ (ḡaraba) to set (of the sun, moon, etc.) and (ḡarb) غَرْب west, occident. Originally this ערב meant to enter, like Ugaritic ʿrb, to enter or Akkadian KU4.RA /erēbu/, to come in, specifically of money, goods, caravan, month, season, water: to come in, to arrive, to flow in. Some say that the word for Arabs developed from this to do business with, to show economic interest, caravan, carriage, as for a people of trade, but the ethonym is too old to be sure.