When Does "X-O Manowar" #0 (1993) Take Place?

"X-O Manowar" #0 v1 "Retribution" Prologue is a science fiction comic-book written by Bob Layton with Jorge Gonzalez and released May 1993. 

It takes place on the fictional Valiant Universe timeline in the year: 

1991 AD 

We know this because the main action of the issue includes a place and time setting textbox midway that reads, "in Earth orbit, November 3, 1991". This is the key date in which the plot of the comic connects back to life on Earth. 

A few pages before that, there is one that says, "Seven years later (time dilation equivalent)." It does not specify whether this is "equivalent" to the time as it's moving on the Spider Aliens' ship or back on Earth. We have to run with 415 AD, as the other option is impossible to calculate without knowing the ship's speed. (Time is relative based on speed, due to gravity, as described by Einstein in the Theory of Relativity.) 

408 + 7 = 415. 

Additionally, there are several prologues within this "Prologue". Each includes a place and time setting textbox of its own. 

The first is declared to be "Ten thousand years ago, on a planet beyond the boundaries of the Milky Way..." . 

1991 - 10,000 = -8009 (i.e. 8009 BC). 

The second is labeled: "Earth, 399 A.D. Region of Southern Europe, Province of Dacia. Inhabited by the Visigoths, August 13, 3:15 P.M." so it's 399 AD at this point. 

Finally, there is a third with yet another textbox that tells us it is "408 A.D. in the village of Rietí in Northern Italy." then one that says, "That Evening." and "Some weeks later. " so we can assume it is still 408 AD. 

It has been collected into a volume 1 hardcover subtitled "Birth" and a volume 1 paperback subtitled "Retribution". 

It is the earliest chronological comic-book in the Valiant Universe (a chronologically first comic to feature X-O Manowar) and can be read at its timeline point, after "Harbinger" #0 (1992) as part of a reading of Valiant Universe prologues, or at the start of "X-O Manowar" v1.