When Does "Metal Gear 2: Solid Snake" (1990) Take Place?
"Metal Gear 2: Solid Snake" is a spy science fiction video game written by Hideo Kojima, designed for the MSX system, that released July 20th, 1990. So when is "Solid Snake" set?
It takes place on the fictional Metal Gear timeline in the year:
We know this because it's specified that Solid Snake is brought out of retirement for this new mission in Zanzibarland on Christmas Eve, 1999 AD so it's December 24th, 1999 AD.

1999 AD
We know this because it's specified that Solid Snake is brought out of retirement for this new mission in Zanzibarland on Christmas Eve, 1999 AD so it's December 24th, 1999 AD.
Also, the game's instruction manual clearly states: "It is 1999", though this is strictly packaging (epitext) it is confirmed by dialogue in the next game.
In "Metal Gear Solid" (1998), Nastasha Romanenko says, "I do not know much about Metal Gear. Just the little I have heard. I know it appeared in Outer Heaven in 1995 and then again in Zanzibar Land in 1999" referring to the events of this game and its predecessor.
It is a chronologically midway game in the Metal Gear universe and can be played either at its timeline point or second [after "Metal Gear" (1987)] as part of a retro release order experience.