When was the sugar-free soft drink which was a later entrant to the diet cola market unveiled 
Diet Pepsi:  Diet Coke was a later entrant to the diet cola market; though shortly after entering production in 1982 it became the primary competing diet cola to Diet Pepsi. Diet Coke:  Unveiled on July 8, 1982 and introduced in the United States on August 9, it was the first new brand since 1886 to use the Coca-Cola trademark.
July 8, 1982