When Does the Novel "Metal Gear Solid" (2008) Take Place?
"Metal Gear Solid" is a spy science fiction novel written by Raymond Benson [adapting "Metal Gear Solid" (1998) by Hideo Kojima with Tomokazu Fukushima] that released May 27th, 2008. So when is "Metal Gear Solid" set?
It takes place on an alternate timeline of the fictional Metal Gear universe in the year:
We know this because early on in chapter two the narrator tells us "Solid Snake checked the codec and verified his GPS position" and the United States of America's Global Positioning System launched its first functioning satellite on February 22nd, 1978 AD (the system was called Navstar at the time). So it must be after 1978 AD.

2005 AD
We know this because early on in chapter two the narrator tells us "Solid Snake checked the codec and verified his GPS position" and the United States of America's Global Positioning System launched its first functioning satellite on February 22nd, 1978 AD (the system was called Navstar at the time). So it must be after 1978 AD.
More specifically, during the second chapter the narrator tells us a character, "rubbed the stubble on his chin. He suddenly felt much older than his thirty-three years." And former Colonel Roy Campbell tells Snake about a specific character; that "He fought in the Gulf War as a teenager, the youngest person in the SAS.", referring to a character who is revealed to be the twin of the "thirty-three"-year-old character rubbing his chin. The Persian Gulf War was a real-world military conflict fought from August of 1990 AD to February of 1991 AD. The youngest a person can join the British military and be eligible to transfer to the Special Air Service (SAS) is 18-years-old. So it must be 2005 AD or 2006 AD.
33 - 18 = 15
and
1990 + 15 = 2005
or
1991 + 15 = 2006.
Although a specific year is not mentioned, the work it's an adaptation of does have a clear time setting for the start of the narrative. The original video game takes place in late February of 2005 AD. Assuming a close adaptation, February of 2005 AD is the best we can ascertain for the story.
Additionally, the first chapter is a prologue (only labeled as "Some Years Ago" with a subheading) depicting a conversation between the then-current "U.S. president", a Dr. Clark Quietly, and a General Jim Houseman about a secret military research project involving a pregnant woman currently giving birth to twins. These twins are revealed to be the character Colonel Campbell is talking about and the one rubbing his chin in chapter two. Therefore this prologue takes place in 1972 AD.
2005 - 33 = 1972.

