When Does "Metal Gear Acid" (2004) Take Place?
"Metaru Gia Ashiddo" (released to English-speaking audiences as "Metal Gear Acid") is a spy video game written by Moku Tochibori with Shuyo Murata and Tomokazu Fukushima designed for the PlayStation Portable that released December 16th, 2004. So when is "Metal Gear Acid" set?
It takes place on an alternate timeline of the fictional Metal Gear universe in the year:
We know this because the dolls use vecuronium bromide, a substance first synthesized in 1973 AD, to paralyze the passengers of Flight 326. So it must be after 1973 AD.

2016? AD
We know this because the dolls use vecuronium bromide, a substance first synthesized in 1973 AD, to paralyze the passengers of Flight 326. So it must be after 1973 AD.
More specifically, United States of American Senator Viggo Hach is referred to by Colonel Roger McCoy as a "presidential candidate in the upcoming elections." and Lena later says, "But, Senator Hach, please speak with caution." and then "The election is just around the corner". So presumably we're in a year with an American presidential election as in the real world.