c. 15 billion years ago The Big Bang, beginning of the Universe (or Multiverse).
c. 6 billion years ago The Guardian of Forever was created. (Some suggest it was created by the race which would eventually evolve into the Q Continuum, but this is highly speculative at best.)
c. 4 billion years ago The first humanoid civilization to evolve in our galaxy explored the cosmos, and, finding themselves alone, seeded the oceans of many M-class worlds with genetic material. These worlds included Earth, Vulcan, Qo'noS, Cardassia, and others.
c. 2 billion years ago A civilization flourished on the planet Tagus III.
c. 100 million years ago A civilization in the D'Arsay system launched an archive of their culture.
c. "millions of years ago" The Organians evolved from humanoids to noncorporeal beings.
c. 600,000 years ago The Tkon Empire became extinct when their sun went supernova.
Around the same time, Sargon's people explored and colonized the galaxy.
c. 500,000 years ago Civilization flourished on Bajor.
Around the same time, the civilization on Exo III moved underground as their sun faded, rendering their planet's surface too cold for habitation.
Also around the same time, the atmosphere of Sargon's planet was ripped away, and the only survivors were consciousnesses suspended in spherical canisters.
At roughly the same time, give or take a few hundred thousand years, the Talosians moved underground after war made their planet's surface uninhabitable; they began to develop their mental powers.
c. 200,000 years ago The Iconian Empire, which utilized inter-dimensional gateways that allowed them to transport instantaneously across interstellar distances, collapsed when all major cities on Iconia were heavily damaged by orbital bombardment.
Around the same time, two unknown adversaries from outside our galaxy fought a war culminating in the creation of at least one "doomsday machine," which eats entire planets.
c. 45,000 years ago Beings from the Delta Quadrant voyaged for over two generations to the planet Earth, making contact with humans on the Siberian Peninsula. The humans called them "Sky Spirits."
c. 25,000 years ago The Trill hosts and symbionts began living as a joined species.
c. 10,000 years ago The Q Continuum began a new era of discovery.
Around the same time, Gint, the first Ferengi Grand Nagus, wrote the Rules of Acquisition.
Also around that time, the first "Tear of the Prophets" was discovered by Bajorans. A total of nine of these artifacts would be discovered over ten millennia.
c. 6000 years ago Unknown aliens took several humans from Earth to be raised on a distant planet. The descendants of these humans were trained to be returned to Earth to prevent the world from destroying itself.
c. 5000 years ago A group of aliens settled on Earth in the Mediterranean area and were worshipped by the ancient Greeks as gods.
3834 B.C. Flint was born in Mesopotamia, Earth.
c. 400 B.C. Aliens whose sun had gone nova visited Earth, and 200 years later settled on a planet they named Platonius, attempting to model it after Plato's Republic.
c. 300-200 B.C. Some evidence suggests that the modern Centaurian civilization is descended from ancient Greeks who may have been transplanted by aliens to Alpha Centauri VII around this time.
c. 2000 years ago Surak led the Vulcan Reformation (or Time of Awakening). Around this time a group who opposed Surak's teachings left Vulcan, and would later found the Romulan Star Empire.
370 Vidiians contracted the Phage.
372 Changelings founded the Dominion.
c. 839 Kahless united the Klingon Homeworld.
1371 The Nacene, explorers from another galaxy, visited the Ocampa homeworld, unintentionally creating an environmental catastrophe. Two of them stayed to create an underground city for the Ocampa, as well as a spaceborn Array to supply power to the city.
1372 The Hur'q invaded Qo'noS; stole the Sword of Kahless.
c. 1570 Bajorans used solar sailing spacecraft.
c. 1700s The Preservers visited Earth. They transplanted a group of Native Americans to a distant planet.
1871 The Cardassian Union was established, uniting the Cardassians under the Detapa Council.
1937 Over 300 Terrans were transplanted to a planet in the Delta quadrant by the Briori.
1947 Quark, Rom, Nog, and Odo crash landed in Roswell, NM, Earth.
1968 Gary Seven, a human who had been trained by unknown aliens (see "6000 years ago"), was returned to Earth to help the planet's inhabitants survive their Nuclear Age.
1977 Space shuttle Enterprise underwent flight testing.
1992 Khan Noonien Singh rose to power, controlling 1/4 of the Earth.
1993 A group of eugenically bred "supermen" seized control in 40 Earth nations, beginning the Eugenics Wars.
1996 Eugenics Wars ended; Khan & 96 fellow "supermen" escaped Earth on sleeper ship S.S. Botany Bay.
1999 Voyager 6 was launched from Earth. It would eventually return as "V'Ger."
2002 Nomad was launched from Earth.
2020 US government created Sanctuary Discticts to shelter and care for the homeless; they quickly degenerated into internment camps.
2024 Reunification of Ireland, March 4.
Bell Riots, Sept. 1-2, in Sanctuary District A in San Francisco. Residents took over an administrative processing center, holding six employees hostage. There they gained access to the planetary computer network, which they used to broadcast their plight to the outside world, from whom injust conditions inside the Districts had been kept secret. When the governor of California ordered Federal troops to retake the processing center by force, hundreds of Sanctuary residents were killed, but resident Gabriel Bell sacrificed his life to save their hostages. The incident would later be named for him. Public opinion turned against the Sanctuary Districts, which were subsequently abolished.
2030 Zefram Cochrane born.
2033 52nd state admitted to the USA.
2036 New United Nations ruled that Earth citizens may not be held responsible for the crimes of their ancestors.
2040 The Death of Television
c. 2053 World War III.
2063 First successful demonstration of warp-speed propulsion by Zefram Cochrane on the Phoenix, April 5. This led to the first (official) contact between Earth and aliens (Vulcans, though they wouldn't have been called that as yet).
2069 The last Klingon emporor died; the High Council was now in control of the Empire.
2072 Suspiria (a Nacene) and 2000 Ocampa left the Ocampa homeworld and established a second Array. There she helped them develop their psychokinetic abilities.
A Q who would come to be called Quinn was imprisoned in a comet.
2079 Around this time, the legal system on Earth seriously sucked.
2103 Mars was colonized.
2113 A united world government was established on Earth.
2151 Earth starship Enterprise embarked on a mission of exploration.
First contact between Earth and the Klingon Empire.
First contact between Earth and the Klingon Empire.
2156-2160 Romulan Wars
2161 United Federation of Planets formed.
Starfleet Academy established.
2165 Sarek was born.
Guinan had her last contact with Q prior to 2365.
2170 Katowa led a group of Native Americans on search for a planet to colonize.
2172 A Class-M planet in the Teplan system in the Gamma quadrant suffered orbital bombardment by the Jem'Hadar as an example to other worlds who would resist the Dominion.
2209 The first case of transporter psychosis was diagnosed by researchers on Delinia II.
2222 Montgomery Scott born.
2223 Relations with the Klingon Empire began to deteriorate (not that they were great before).
2227 Leonard H. McCoy born.
2229 Marriage of Amanda Grayson and Sarek.
2230 Spock born.
2233 James T. Kirk born, March 22, in (unofficially) Riverside, Iowa.
2237 Hikaru Sulu born.
2239 Nyota Uhura born.
2245 Federation starship Enterprise launched, under Captain Robert April.
Pavel A. Chekov born.
2250 Vulcanian Expedition. Starfleet sends four starships to Vulcan to remind them of their obligation to provide citizens to serve in Starfleet. This would result in the commissioning of the science vessel Intrepid, crewed entirely by Vulcans.
2251 Enterprise's 2nd 5-year mission began, under Captain Christopher Pike.
2255 The Treaty of Armens signed, ending the conflict between the Federation and the Sheliak Corporate.
2257 Enterprise's 3rd 5-year mission began, under Captain Christopher Pike.
2261 David Marcus born, to Carol Marcus & James Kirk.
2264 Enterprise's 4th 5-year mission began, under Captain James T. Kirk.
2265 El Aurian homeworld destroyed by Borg.
Stuff in darkred text has to do with the series "Enterprise," which may or may not be accepted as officially part of the Star Trek universe.
Stuff in black text can generally be assumed to be part of official Star Trek, usually if not always taken from the Chronology (Michael and Denise Okuda), startrek.com, or the various shows and movies.