To understand completely the whys and wherefores of the Dune Universe one needs to learn about its past. If you have read the books and seen both the Dino De Laurentis/David Lynch film "Dune" and the recent Sci-Fi Channel production "Frank Herbert's Dune" you may be quite familiar with the story and understand what consists of the Imperium, but may not be sure of how the Imperium became the Imperium. Or you may be a "Dune virgin" and it all seems Greek to you. Whether an old hand or a newbie to the story I think it is essential to get behind the story of the Dune Chronicles as it occurs in the novels. If you are like me, learning such things about a story that has captured one's attention makes it more fun!
First of all, the timeline of the stories of Dune occurs 24,360 years into the future! That is a grandiose timeframe indeed! Frank Herbert had imagined a LOT had happened in the universe he created between our time and the time of the characters he made. One may ask how I have come to this figure. Well, it was made easy for me in The Dune Encyclopedia, compiled by Willis E. McNelly and published by Berkeley Books, New York (copyright: 1984); in it was published a chronology of the events that occurred before and after the arrival of Paul Atreides to the scene of the Imperium. What I had done was to use a known historical person, Alexander III of Macedon (Alexander the Great), who was listed in the chronology, and extrapolated everything from his time (c. 356-323 BCE), using the year 330 BCE (when Alexander defeated the Persian King Darius in battle) as the keystone for calculating the entire chronology.
Before I go on, I would like to explain briefly the meanings of those shortcuts to chronological dates. For our real universe, we moderns use the Western designations BC (Before Christ) and AD (anno domini, Latin for "year of our Lord"), which is religiously based, to denote years of the past; the secular version of BC and AD are BCE (Before the Common Era) and CE (Common Era), BCE taking the place of BC and CE in the place of AD. I prefer to use the BCE/CE secular chronology terms.
It also occurs that Frank Herbert placed into his universe chronological terms that the reader comes across which needs explanation. You will neither find BC/AD nor BCE/CE designations for the past in the Dune Universe. What one does find is BG and AG. The keystone for these chronological terms is one event that occurred in the Dune Universe which had established the Imperium as the characters of the story know it: the coalescing of three organizations into one power structure, which would be the year 0 (zero). Everything before that is BG, BEFORE GUILD (Before the Guild), and everything after that year is AG, AFTER GUILD; in other words before and after the foundation of the Spacing Guild's monopoly on space travel.
One last thing: the year of the events that take place in the novel Dune is given as AG 10,191. According to my calculations, that year would be placed in the year CE 26,361: 24,360 years into our future!
A Chronology of the Dune Universe
Dune Encyclopedia