When Does "Metal Gear" (1987) Take Place?

"Metal Gear" is a spy video game written by Hideo Kojima, designed for the MSX system, and released July 1987. 

It takes place on the fictional Metal Gear timeline in the year: 

1995 AD 

We know this because "Metal Gear 2: Solid Snake" (1990), the second installment of the franchise, takes place in December of 1999 AD and specifically refers to the infiltration of Outer Heaven and other events of this game as taking place 3 years in the past, which would be 1996 AD (this was later changed to 4 years in the past in the mission files readable in "Metal Gear Solid" (1998), the third installment of the franchise and reaffirmed in various timelines over the following games). 

1999 - 3 = 1996. 

or 

1999 - 4 = 1995. 

It is a chronologically midway game in the Metal Gear universe and it can be played either at its timeline point or first [before "Metal Gear 2: Solid Snake" (1990)] as part of a retro release order experience.