AD - Anno Domini and BC - Before Christ: The most widely used system on modern Earth, supposedly dated from the birth of Jesus Christ in AD 1 (Anno Domini means "Year of our Lord"). This calculation was later found to be off by a few years, but AD 1 has remained the same. Years prior to AD 1 are in "BC," beginning with 1 BC and working backwards from there. AD and BC are known more secularly as "CE" (Common Era) and "BCE" (Before the Common Era).
CY - Commonwealth Year and BIE - Before Imperial Era: Dating of the Systems Commonwealth in Andromeda, dated from 0 CY (Coronation Year) (4921 BC) when the Vedran Empire was founded. This later became "Commonwealth Year." Years before 0 CY are designated as "Before Imperial Era" or BIE.
B.G. - Before Guild and A.G. - After Guild: Dating system used in the future world of Dune. Around AD 13,000, the A.G. system began with 1 A.G. Years before 1 A.G. are listed as B.G., and work backwards from there; c. 11,000 BG is around AD 2000.
UC - Universal Century and CC - Correct Century: System used in the Gundam "Universal Century" timeline, beginning with UC 0001 (AD 2081) with the completion of the first space colony. By the year 2320 (AD 4400), the system's name had switched from "UC" to "CC."
A.L. - Aeon Lanack and A.R. - Aeon Robotech: System used in the Robotech universe by the Robotech Masters (and the Zentraedi). Each year in that system equals one-fourth of our years; in other words, four of their years pass for every one of ours. Aeon Lanack became Aeon Robotech in 1936 A.L. (1 A.R.). The link to Earth dating occurs in 671 A.R., which was in December AD 1999. Thus, AD 1999 spanned the years 668 to 671 A.R.
B.B.Y. - Before the Battle of Yavin and A.B.Y. - After the Battle of Yavin: System used in the Star Wars universe, dated from the Battle of Yavin (the first major rebel victory) in 0 B.B.Y. (AD 1815). Years before work backwards in B.B.Y., years after work forwards in A.B.Y.
First Age, Second Age, Third Age, Fourth Age: Dating system used in Middle-Earth (Lord of the Rings). The First Age (I 1) begins c. 11,084 BC; the Second Age (II 1) begins c. 10,501 BC; the Third Age (III 1) begins c. 7061 BC; and the Fourth Age begins (IV 1) c. 4041 BC.
S.R. - Shire Reckoning: Dating system used in the Shire (Lord of the Rings). S.R. 1, the founding of the Shire, is the same as III 1601 (c. 5461 BC).
Warhammer 40,000: Uses the same dating as AD, but in a different format. The current millennium is key in their era. For example, AD 2004 is in the third millennium AD (M3), year 004, and would be formatted 004.M3.
P.A. - Post Apocalypse: A dating system established in the Rifts future, after a period of dark ages following the opening of the rifts. It begins at 1 PA in the year AD 2287.
A.L. - Anno Liberatis: In the alternate history Gallatin Universe, the fledgling United States retroactively established 0 A.L. at 1776 C.E., the year of the Declaration of Independence. Dates before that continue to use the A.D. system's numbers, but are instead designated C.E. (Christian Era).
Y: In the Star Fleet Universe, years are dated from Y1, humanity's first contact with an alien race (the Vulcans). Y1 is equivalent to AD 2112, based on the imposition of the Organian Peace Treaty (Y156/AD 2267).
Real World Link Dates
Andromeda: According to "A Commonwealth Timeline" on the official Andromeda site and message board posts citing Andromeda writer Zack Stentz, 6811 CY is the same as AD 1891, the year the Thomas Common English translation of Nietzsche's Also sprach Zarathustra (Thus Spake Zarathustra) stated as the original publication date for the work. (It was actually the year Nietzsche's family delayed the fourth part's release to, although the public version was really available in 1892.) (6811 CY = AD 1891)
Dune: According to the official Dune novels timeline, c. 11,000 B.G. (Before Guild) is c. AD 2000, while 1 A.G. (After Guild) is c. AD 13,000. Different link dates were provided in The Dune Encyclopedia, but that work has been declared non-canon in more recent years. (c. 11,000 BG = c. AD 2000; 1 AG = c. AD 13,000)
Gundam: UC 0001 is AD 2081, as meticulously determined by this site. I also subscribe to the extremely probable belief that "Correct Century" is a continuation of "Universal Century," so CC 2345 is AD 4425. (UC 0001 = AD 2081)
Robotech: As derived from Robotech: The Complete Handbook and the data collected here, 671 A.R. (Aeon Robotech, four years pass to our one) was in December AD 1999, which means 1999 spanned the years 668 to 671 A.R. Aeon Robotech switched from Aeon Lanack (A.L.) in 1936 A.L. (668 to 671 A.R. = AD 1999)
Star Wars: Star Wars was, until recently, completely incomparable to Earth standards. However, information from an alternate-universe story in Star Wars Tales #19 makes it possible to date the original film (0 B.B.Y., Before the Battle of Yavin) at c. AD 1815. (0 BBY = c. AD 1815; 1 ABY = c. AD 1816)
Lord of the Rings: Middle-Earth's timeline is broken into four (known) Ages, and takes place as some inspecific date in Earth's prehistoric past. However, a letter from J.R.R. Tolkien in 1958 dated the end of the War of the Ring at about 6000 years prior; so year 3019 of the Third Age is about 4043 BC. Extrapolating from that, year 1 of the First Age is c. 11,084 BC; year c. 584 of the First Age/year 1 of the Second Age is c. 10,501 BC; year 3441 of the Second Age/year 1 of the Third Age is c. 7061 BC; and year 3021 of the Third Age/year 1 of the Fourth Age is c. 4041 BC.
Rifts: As implied by the Triax and the NGR book and confirmed in the Rifts FAQ on the Palladium site, 1 PA (Post Apocalypse) is the same as AD 2287. (1 PA = AD 2287)
Gallatin Universe: As explicitly stated in The Probability Broach's timeline, 0 A.L. (Anno Liberatus) is the same as 1776 C.E. (AD 1776). (0 A.L. = 1776 CE = AD 1776)
Star Fleet Universe: The history of the Star Fleet Universe differs from that of the mainstream Star Trek universe, but there is one commonality- the Organian Peace Treaty imposed on the Federation and Klingons. According to the Star Trek Chronology, the Organians imposed the Treaty in 2267; in the Star Fleet Universe timeline in GURPS Prime Directive, this event occurred in Y156. (Y156 = 2267)