When Does "The World of Krypton" #1 (1987) Take Place?

"The World of Krypton" #1 v2 "Pieces" is a science fiction comic-book written by John Byrne and released August 11th, 1987. 

It takes place on an alternate timeline of the fictional DC Comics Universe in the year: 

99046 BC 

We know this because narration in "The World of Krypton" #3 v2 tells us the events of issue #1 are a framed history “a thousand centuries past” while showing Jor-El (Kal-El/Superman’s father) watching hologram records of them. The plot continues into #4, but that part of the story only specifies one scene as taking place “months ago” so it seems likely that entire issue takes place within one year of when the calendar year Kal-El/Clark Kent/Superman is sent to Earth as a baby. 

"Man of Steel" #1 v1 (1986) and “The World of Smallville” #2 (1988) establish that year as 1954 AD. 

A thousand centuries is: 

100 x 1,000 = 100,000 

so 

1954 - 100,000 = -98,046. 

And, in "The World of Krypton" #2 v2 (1987), an omnipotent narration textbox tells us (from Van-L’s point-of-view), “Even after a thousand years…” while referring to something that happens halfway through that issue– in a scene that takes place on the same day as this issue's entire plot. 

-98,046 - 1,000 = -99,046. 

Later, in "The World of Krypton" #3 v2 (1987), the events of this issue are also referred to by an A.I. as occurring “in the year 10⁵/892 of the Fifth Age”. Presumably, 10⁵/892 of the Fifth Age is the equivalent year on a Kryptonian calendar. 

This comic-book was retroactively collected into a "Superman: The World of Krypton" paperback and then collected again into a later, expanded "Superman: The Many Worlds of Krypton" paperback. 

It is the chronologically earliest comic in the DC Comics Universe multiverse timeline (a chronological first comic to feature Kryptonians).