The Universal Standard Calendar is the standard measurement of time in the known universe. It centered around the founding of the Spacing Guild as the starting year. 'BG stands for Before Guild, and AG stands for After Guild.
Behind the scenes
The Dune novels use a different calendar from that of present day Earth. Years are not counted before and after the birth of Christ, but before and after the formation of the Spacing Guild, measured as before and after Guild.
Using the information given by the Dune books, it is possible to determine how the dating system used in the novels corresponds to our own.
1. Butlerian Jihad
Butlerian Jihad lasts "two cruel generations" Dune, Appendix II / V
201 - 108 BG
2. The 200th century
"Mankind's movement through deep space placed a unique stamp on religion during the one hundred and ten centuries that preceded the Butlerian Jihad. " Dune, Appendix II
~11,200 years
Dune begins in 10,191 AG, so we simply add 10,191 to 11,200+201 together:
Thus, the year 10,191 AG corresponds to the year 21,391 AD. That is, of course, assuming that the Dune chronology actually uses Earth years.
But Dune counts in Standard years:
- "Taraza was momentarily abashed. This was an imposition. Teg was still a regal figure, tall and with that large head topped by gray hair. He was, she knew, four SY short of three hundred. Granting that the Standard Year was some twenty hours less than the so-called primitive year, it was still an impressive age with experiences in Bene Gesserit service that demanded that she respect him." HoD ~Page 36
So there could be a maximum error of about 400 years (since we do not know when the Dune Universe started using the "Standard" year). This method also assumes that humankind began exploring "deep" space in the 20th century a decade after World War II.