Question: When was the sugar-free soft drink which was a later entrant to the diet cola market unveiled 

Context paragraphs:
Jolly Cola: Jolly Cola is an original Danish soft drink dating back to 1959.  Jolly Cola is now produced by the Danish brewery ‘Vestfyen’, which also produces Jolly Light, the sports drink Jolly Time and Jolly Orange.  Up until the 1980s, Jolly Cola had a market share of about 40% of the Danish cola market.  This was extraordinary, as Denmark is one of a few countries in the world, where another cola than the original Coca-Cola has had a larger market share.

Diet Pepsi: Diet Pepsi and Diet Pepsi Classic Formula Blend (stylized as diet PEPSI CLASSIC SWEETENER BLEND) are no-calorie carbonated cola soft drinks produced by PepsiCo, introduced in 1964 as a variant of Pepsi-Cola with no sugar.  First test marketed in 1963 under the name Patio Diet Cola, it was re-branded as "Diet Pepsi" the following year, becoming the first diet cola to be distributed on a national scale in the United States.  In the 1960s and 1970s its competition consisted of Tab, produced by The Coca-Cola Company, and Diet Rite soda, produced by Royal Crown.  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.

Tab (drink): Tab (stylized as TaB) is a diet cola soft drink produced by The Coca-Cola Company, introduced in 1963.  The soda was popular throughout the 1960s and 1970s, and several variations were made, including Tab Clear as well as caffeine-free versions.

Diet Coke: Diet Coke (called Coca-Cola Light in some countries) is a sugar-free soft drink produced and distributed by The Coca-Cola Company.  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.  The product quickly overtook the company's existing diet cola, Tab, in sales.

Caffeine-Free Pepsi: Caffeine-Free Pepsi-Cola was introduced, under the brand name Pepsi Free, in 1982 by PepsiCo as the first major-brand caffeine-free cola in years.  (The Canada Dry Company had marketed "Sport Cola," which was also caffeine-free, in the 1960s, but that brand of cola was not a success.)  A sugar-free variant, then known as Diet Pepsi Free, was also introduced.  The Pepsi Free name itself was phased out in 1987, and today these colas are known simply as Caffeine-Free Pepsi and Caffeine-Free Diet Pepsi.  The term "Free" had been reintroduced by PepsiCo on their Sierra Mist line in response to Coca-Cola's successful introduction of their Zero line (although the "Free" refers to a lack of sugar rather than caffeine, since Sierra Mist is already caffeine-free), though in 2008 that drink had reverted to its previous name of Diet Sierra Mist.

Cola chicken: Cola chicken is a chicken dish prepared using chicken and cola soft drink as main ingredients.  The cola is typically mixed with another ingredient, such as soy sauce, barbecue sauce or ketchup.  It can be prepared with regular or diet cola.  As the dish cooks, the sauce reduces, accentuating the cola flavor and creating a glaze in the process.  It is sometimes prepared as a chicken wing dish.  It can have a sticky texture, depending on how it is prepared.  Cola chicken has been described as a dish that has flavor elements of sweet and sour, and the cola has been described as imparting a rich flavor in the chicken meat.

Fentimans Curiosity Cola: Fentimans Curiosity Cola is a botanically brewed carbonated cola soft drink, a product of British brewery Fentimans.  The brewing process used gives the drink an alcohol content of up to 0.5%, but this small amount still allows it to be sold as a soft drink.  It is sold in 275ml and 750ml glass bottles.  In Europe and North America it is legal to sell the cola to minors.

Patio (soda): Patio Diet Cola was a brand of diet soda introduced by Pepsi in 1963.  It was created in response to Diet Rite Cola.  Fitness promoter Debbie Drake was Patio Diet Cola's spokesperson; the drink was also marketed as a soda alternative for diabetics.

Piscola: Piscola or Combinado Nacional (national mix) is a highball cocktail, made of pisco and most commonly a cola drink, that is popular in Chile.  A piscola may be "black" or "white" depending if it is mixed with a cola or ginger ale, Tonic, Sprite or a similar soft drink.  The drink is prepared by filling a highball glass with ice and then adding pisco followed by the soft drink in a proportion ranging from 1:1 to 1:3.  Sometimes slices of lemon or key lime may be added.  In Chile, February 8 has been celebrated as the "day of the piscola" since 2003.

Like Cola: Like Cola was an unsuccessful cola soft drink, introduced by the 7 Up company (while owned by Philip Morris), that appeared on the American market in 1982.  Its slogan was "Made From The Cola Nut."  Like Cola was one of the first attempts at a cola without caffeine.  It was packaged in a red and blue can.  A diet version was also available, with the color scheme reversed.

Supporting paragraphs: Diet Pepsi Diet Coke
Answer: July 8, 1982