Well, now, Brother William, when the house of Israel begin to come into the glorious mysteries of the kingdom, and find that Jesus Christ, whose goings forth, as the prophets said, have been from of old, from eternity: and that eternity, agreeably to the records found in the catacombs of Egypt, has been going on in this system, (not this world) almost two thousand five hundred and fifty five millions of years: and to know at the same time, that deists, geologists and others are trying to prove that matter must have existed hundreds of thousands of years;-it almost tempts the flesh to fly to God, or muster faith like Enoch to be translated and see and know as we are seen and known!
This quote, which dates the origins of the universe-- "not the world"-- to almost 2,555,000,000 years ago, has occasionally been lauded as something of a scientific home run for Joseph Smith. (Never mind that current projections make the age of the universe more like 14 billion years.) More interesting to me is that in "Worlds Without Number: The Astronomy of Enoch, Abraham, and Moses" (BYU Studies 8:3), Grant Athay cited it as evidence that Joseph Smith translated more of the Book of Abraham than he published. In fact, however, the figure cited by Phelps comes from the extant Book of Abraham.
As I'm sure you all know, Abr. 3:4 indicates "that Kolob was after the manner of the Lord, according to its times and seasons in the revolutions thereof; that one revolution was a day unto the Lord, after his manner of reckoning, it being one thousand years according to the time appointed unto that whereon thou standest. This is the reckoning of the Lord’s time, according to the reckoning of Kolob." You will also recall that D&C 77 speaks of "the seven thousand years of [the] continuance" of the earth. Phelps appears to have arrived at his figure by multiplying these numbers together. 7,000 Earth-years x 365 days-per-Earth-year x 1000 Kolob-years-per-Earth-day happens to give us a figure of exactly 2,555,000,000 Kolob-years. Apparently Phelps assumed that the earth was almost at the end of its 7,000-year lifespan and that he was therefore safe in using this figure to calculate the approximate age of the universe.
It's worth adding that Phelps appears to misunderstand the logic of planetary revolutions and the reckoning of time. He seems to assume that a good deal more time has passed on Kolob than on earth. In reality, the differences in the speed of planetary revolution would result in the same amount of time merely being reckoned differently. Phelps' misguided calculation is ultimately little more than an irrelevant-- albeit interesting-- footnote in Mormon history.
-Chris