When Does "Metal Gear Solid V: The Phantom Pain" (2015) Take Place?
"Metaru Gia Soriddo Faibu Fantomu Pein" (released to English-speaking audiences as "Metal Gear Solid V: The Phantom Pain") is a spy science fiction video game written by Hideo Kojima with Shuyo Murata, Hidenari Inamura, and Etsu Tamari, designed for the PlayStation 3 and PlayStation 4 systems, that released September 1st, 2015. So when is "The Phantom Pain" set?
It takes place on the fictional Metal Gear timeline in the year:
We know this because Revolver Ocelot says, "9 years ago in that helicopter", referring to the intense climax of the previous game, "Metal Gear Solid V: Ground Zeroes" (2014), which takes place in March of 1975 AD.

1984 AD
We know this because Revolver Ocelot says, "9 years ago in that helicopter", referring to the intense climax of the previous game, "Metal Gear Solid V: Ground Zeroes" (2014), which takes place in March of 1975 AD.
3.1,975 + 9 = 3.1,984.
He even says, ..."during Operation Snake Eater 20 years ago" at one point, referring to the events of "Metal Gear Solid 3: Snake Eater" (2004), which take place in 1964 AD.
1,964 + 20 = 1,984.
More specifically, he also says, "4 years ago the Soviets invaded Afghanistan.", referring to Operation Baikal-79, which happened in the real world on December 27th, 1979 AD. Exact math would make it December of 1983 AD but presumably Ocelot is rounding up. This indicates it is early 1984 AD.
12.1,979 + 4 = 12.1,983.
Specifically, a time and place setting title card appears over the game graphics very early reading: "Dhekelia SBA Memorial Hospital, Cyprus March 11th, 1984 02:03". So it is most likely March of 1984 AD at the beginning of the main action.
Additionally, the game begins with three short prologues labeled with their own relative time setting subtitles and the last one reads: "2 weeks after awakening". So the very first is almost certainly in late February of 1984 AD.
It was adapted to prose in late 2015.

