Here is a complete timeline of all the major events that occurred before the Dragonball series began, and up until the very end of Dragonball GT. (Note: All these dates are assumed to occur in A.D., except otherwise noted.)

Ultimate Dragonball Z/GT Timeline

The timeline is ordered by the following:

Preface (Before Dragonball starts)
Over 100 million B.C. Earliest Kai is thought to exist
Around 75 million B.C. The 15th generation Supreme Kai is sealed in the Z Sword
Around 5 million B.C. Bibidi creates Buu; Buu destroys hundreds of planets and kills 4 Kaioushins. Buu absorbs Dai Kaiou, making him seem fat.  West Kaikills Bibidi and leaves Buu's sphere shell on Eart.
50-10 thousand B.C. Annin begins protecting the Furnace of Eight Divinations on the border between Real world and the Other world
4237 B.C. Garlic Junior's ancestors migrate to Earth
739 B.C. Jadoushin becomes the Princess of Serpent Path
238 B.C. The Legendary Super Saiya-jin begins destroying the universe
250 A.D. Usanai Baba begins her fortune telling business
261 King Yemma travels Snake Path to get training from King Kai.  Princess Snake sees him, and falls in love. A violent storm begins on Namek; Piccolo is still a child and is sent off the planet.  He later arrives on Earth
430 Master Roshi (Muten Roshi) is born
459 Tsurusennin's brother, Tao Pai Pai is born
461 Kami selects Piccolo as his successor; Garlic assaults Kami for not selecting him, so Kami traps him.  Piccolo removes the evil from his heart, creating Piccolo Daimaou.  Daimaou terrorizes Earth, and gets sealed in the Denshi Jar by Mutaito .  This causes Mutaito to die.
474 Darbura watches Earth
550 The first Saiya-jin lands on Planet Plant
553 Master Roshi begins looking at dirty magazines
650 Master Roshi finds the 3-Starred Dragonball washed up on his island.  The first Tenkaichi Budoukai occurs.
658 Son Gohan Ojisan is born
662 Kami tells Mr. Popo his childhood memories
698 Toninjinka is born
715 Pilaf is born
720-730 War between the Truffles and Saiya-jin race begins.  The Saiya-jin defeat the Truffles and rename Planet Plant to Planet Vegeta.  They then adopt the Truffles technologies.
722 General blue from the Red Ribbon Army is born
730 Tao Pai Pai starts his career as an assassin
731 Saiya-jin makes a treaty with Freeza and start capturing planets and selling them.  King Vegeta marries his queen
732 Prince Vegeta is born
733 Bulma, Yamcha, and Tien are born.  Earth's current king is sworn in.
735 Freeza starts to feel misgivings toward the Saiya-jin
736 Krillin is born
737 Kakarot is born; his father rejects him as a failure because he's a low-class Saiya-jin.  Planet Vegeta takes control of Planet Meat and Kanassa.  Bardock has premonitions of Goku fighting Freeza.  King Vegeta dies while revolting against Freeza.  Freeza destroys Planet Vegeta. Goku is sent off of Planet Vegeta to Earth. Chi-chi is born.
738 Goku hits his head on a rock after falling into a canyon. This causes him to loose his memory and mission to destroy Earth.
739 Ox King and his daughter becomes trapped in Mount Frypan
740 Oolong is born.  Krillin begins his training at the Orinji Temple. Bebi creates Dr. Myu to gather energy so he can be reborn
747 Yamcha meets Puar
748 Master Roshi's Phoenix dies of food poisoning. He discovers a lost baby sea turtle
749 Bulma enters Senior High School in the Western City.  She discovers the 2-Star Dragonball in her basement. On August 22, Bulma discovers the 5-Star Dragonball in a cave to the North
Beginning of Dragonball Series: Pilaf Saga
Sept. 1,  749 On summer vacation, Bulma meets Goku. Goku shows her the ball which he thinks contains his grandfather's soul.  This ball is actually the 4-Star dragonball.
Sep. 2, 749 Bulma nd Goku meet Master Roshi; Master Roshi gives them the 3-Star dragonball in exchange to touch Bulma's breasts.
Sept. 5, 749 Goku stops Oolong's reign of terror and an old women gives him the 6-Star dragonball for the deed.
Sept. 6, 749 Goku meets Yamcha and fights him; the battle ends in a draw
Sept 9, 749 Goku defeats Yamcha.  Master Roshi uses a Kamehameha to put out the flames on Frypan Mountain, unfortunately, he destroys the mountain and the castle.  In the rubble, they find the 7-Star dragonball.  Goku defeats Monster Carrot and his gang.  Shuu and mai steal all but one of the dragonballs from Goku and the gang. Pilaf summons Shenlong, but Goku sees the full moon and turns Oozaru; then he destroys Pilaf's castle 
Dragonball: Red Ribbon Army Saga
Sept. 10, 749 Goku and Krillin becomes Master Roshi's students
Sept. 14, 749 Goku and Krillin begin their training.  They are forced to wake at 4:30 in the morning everyday to train
Oct. 2, 749 The second term of Western City Senior High School begins
April 6, 750 Goku and Krillin increase the weights on their backs to 40 pounds to further train
April 18, 750 Commander Silver of the Red Ribbon Army begins searching for the dragonballs
May 6, 750 Goku, Krillin and Master Roshi leave to fight in the 21st Tenkaichi Budoukai
May 7, 750 The 21st Tenkaichi Budoukai begins. Jackie Chun places first in the tournament and Goku takes second place. Goku, Pilaf and RR Army fight for the dragonballs.  The RR Army takes the 5-Star and 6-Star dragonballs.  Goku and Chi-chi meets in Ox King's village.
May 8, 750 Goku destroys the RR Army's Muscle Tower in the North
May 9, 750 Bulma fixes Goku's dragon radar. Goku fights Blue Shogun in the sky above Penguin Village. Arale-chan beats Blue Shogun without Goku's help.  Tao Pai Pai kills blue with only his tongue.  He wails on Goku at the base of Karin Tower, which Goku later begins to climb
May 10, 750 Summer vacation begins for Penguin Village High School. Goku reaches the top of Karin Tower and begins his training of taking the Super Holy Water from Karin
May 12, 750 Goku gets the Super Holy Water, which he finds only to be tap water. The actual training was only to get him stronger. Goku kills Tao Pai Pai at the base of Karin Tower.  Goku defeats all of the Red Ribbon Army. Goku is reunited with his dead Grandpa, Gohan.
July, 750 The raining season begins in Namu's village on Namek
Dragonball: Piccolo Saga
750-753 A.D. Goku begins his training in which he runs around the world. In Chao village, he fights Kinkaku and Ginkaku. He meets Chintaiken and fights Tenron in the Gozen-Jiai contest. Goku enters the Demon World and fights the lord of this world, Shura. He saves Inoshikachou and meets Tienshinhan for the first time.
May 5, 753 Goku meets Konkichi. Goku swims to Papaya Island where the 22nd Tenkaichi Budouki is being held
May 7, 753 The 22nd Budoukai begins.  Tien wins and Goku comes in second place. Killin is killed when Piccolo Daimaou appears
May 8, 753 The celebration of the 20 years under the rule of the King of the World
May 9, 753 Goku kills Piccolo Daimaou, and Piccolo is born. Goku reaches Kami's Palace and trains with Mr. Popo and Kami for the next three years.
May 7, 756 The 23rd Budoukai begins. After Chi-chi and Goku fight, they become engaged. Goku beats Piccolo and earns his first win at a Tenkaichi Bodoukai. The planning for Chi-chi's and Goku's wedding begins, but it is interrupted by the eruption of a fire in which Ox King is trapped inside Chi-chi's wedding room along with her dress. Chi-chi and Goku search for the Bashoo Sen to put out these magic flames.
May 8, 756 Grandma Hakkake teaches Chi-Chi how to be a good bride. Goku uses Basho Sen to put out the flames, but it doesn't work. He learns the flames are a result of a leak in the Furnace of Eight Divinations on the opposite side of the world. Goku travels to Mount Gogyou (the home of the Furnace of Eight Divisions), where he once again meets his grandpa. He fixes the hole in the bottom of the furnace and the flames cease around Ox King's castle
May, 757 Grandpa Gohan is born.
759-760 Goku is interviewed by Shonen Jump
760-770 Dr. Myu finds the alien Rirudo and turns him into Machine Mutant Shogun Rirudo. Rirudo uses his great powers to complete the construction of planet M2. The Machine Mutant begins to gather energy to resurrect Bebi.
The Beginning Dragonball Z: Saiya-jin Saga
Oct. 12, 761 Radditz arrives on Earth; Goku and Radditz die from Piccolo's Special Beam Cannon. Vegeta and Nappa land on planet Arlia and they are heading for Earth.
Period of Several Months Gohan begins his survival training. He finds a destroyed robot and becomes more mature. He tries to escape his training and go home, but when he meets Pigero and other orphans whose parents were killed in a tsunami, he finds enough courage to go back to his training. Yamcha is employed as a profession baseball player.
March 762 Krillin, Yamcha, Tienshinhan, Chaozu and Yajirobe train at Kami's Palace
April 29, 762 Goku arrives at King Kai's planet
May 9, 762 Goku catches Bubbles
May, 762 Krillin, Yamcha, Tienshinhan,and Chaozu make a spiritual travel to old Vegeta. They are easily beaten by the two "weak" Saiya-jin there.
May 23, 762 Goku hits Gregory with the huge hammer
Nov. 2, 762 Goku is resurrected
Nov. 3, 762 Vegeta and Nappa arrive on Earth. Nappa, Yamcha, Piccolo, Chaozu and Tienshinhan are killed. Gohan, Krillin, and Goku defeat Vegeta. Goku convinces Krillin to have mercy on Vegeta. Vegeta blasts off in his space pod. Dr. Gero deploys his micro-sized robot to collect blood samples from the strongest fighters in the world.
Nov. 4, 762 Goku, Krillin ad Gohan are hospitalized in the East City Hospital
Nov. 7, 762 Krillin and Gohan leave the hospital
Nov. 9, 762 The reconstruction of the Namekian ship Piccolo came in is completed
Nov. 14, 762 Bulma completes her lessons in the Namekian language. Bulma, Gohan and Krillin blast off for the destination, Planet Namek.
Nov. 21, 762 Vegeta lands on Freeza Planet Number 79
Dec. 13, 762 Vegeta is healed; he immediately leaves for Namek
Dragonball Z: Freeza Saga
Dec. 18, 762 Vegeta, Kuwi, Bulma and the gang all land on Namek abou the same time. Goku leaves for Namek. Dende takes Krillin to meet Guru. Piccolo, Tien,Chaozu and Yamcha make it to King Kai's planet. Goku's spaceship gets trapped in the gravity of the star, Alpha HZ. He uses Kamehameha to blow himself away.
Dec. 19, 762 Zarbon beats up Vegeta and takes him back to Freeza's ship to heal him.
Dec. 20, 762 Vegeta fights Zarbon once more and kills him. Krillin takes Gohan to meet Guru. Freeza orders the Ginyu Force to arrive on Namek immediately.
Dec. 23, 762 Goku completes hi training under 100x gravity
Dec. 24, 762 The Ginyu Force is killed, except for Ginyu himself. Piccolo is revived, and he fuses with Nail. Krillin is killed by Freeza, and Goku turns into a Super Saiya-jin. A wish is made with the Earth's Dragonballs to have everyone killed by Freeza or his followers wished back to life. With the Namek Dragonballs, everyone on Namek is wished back to Earth, except for Freeza and Goku. Goku defeats Freeza. Namek explodes.
May 3, 763 Krillin and Yamcha are revived with the Namekian dragonballs.
Sept. 9, 763 Tien and Chaozu are revived. The Nameks are wished to New Namek.
Oct., 763 Makyo Star is visible from Earth and Garlic Jr. returns from the Dead Zone. Gohan sends him back to the Dead Zone after destroying the Makyo Star
764 Freeza Cyborg's body is completed. Cell arrives in Trunk's time machine and goes underground to absorb nutrients.
Aug., 764 Trunks arrives and kills Freeza and King Cold. Goku returns to Earth.
Dragonball Z: Cell Saga
764-766 Vegeta trains to become a Super Saiya-jin. A relationship develops between Vegeta and Bulma. Goku and Piccolo attend driving school in order to get their driving licenses. Goku eventually does get it.
766 Trunks is born.
767 Goten is born.
May 7, 767 The 24th Tenkaichi Bodoukai takes place, ending with Hercule as the champion.
May 12, 767 All the artificial humans (Androids) show up. Piccolo and Kami fuse. Goku suffers from his fatal heart disease. Cell of the present day is destroyed along with Dr. Gero's underground lab by Krillin and Trunks.
May 15, 767 Goku's heart disease is cured by Trunk's medicine. Vegeta and Trunks enter the Room of Spirit and Time
May 16, 767 Cell reaches second stage. Cell reaches perfect form. Goku and Gohan train in the Room of Spirit and Time.
May 17, 767 Cell announces the Cell Game to the entire planet. Goku and Gohan exit the Room of Spirit and Time and Piccolo enters.
May 18, 767 Piccolo exits the Room of Spirit and Time, and Vegeta enters again. Gohan saves Lime's village from Barbon.
May 19, 767 Cell destroys the Royal Army. Dende becomes the new guardian of Earth. Vegeta exits the Room of Spirit and Time; Trunks enters.
May 20, 767 Trunks exit the Room of Spirit and Time
May 26, 767 The Cell Game begins at noon. Goku sacrifices himself to prevent Earth from blowing up. Gohan totally destroys Cell.
May 27, 767 Goku's funeral takes place. Trunks goes back to his time.
Dragonball Z: Buu Saga
May-June, 767 Goku participates in the Annoyoichi Budoukai, which is in honor of the North Kaiou's death. Goku fights Pikkon, whose fighting technique is similar to Piccolo's. Trunks takes his first steps.
768 Doll-Taki is employed by Dr. Myu. His task is to gather energy to awaken Ruudo. He does so.
770 Krillin and Android 18 get married.
771 Marron is born.
773 Supreme Kai and Kibito travel to Earth in search of the sphere that holds Buu.
March 28, 774 Gohan completes first grade at Orange Star High School
April 7, 774 A sighting occurs of the Golden Haired Warrior in Satan City. Gohan begins second grade at Orange Star High School and he meets Videl. Bulma creates a transformation suit for Gohan. The Great Saiyaman appears.
April 8, 774 Videl discovers that Gohan is the Great Saiyaman
April 9, 774 Gohan teaches Video how to fly. Southern Kaiou watches Goku's training.
April 10, 774 Videl masters the flying technique
May 8, 774 Piccolo, Gohan and Gotenks are absorbed by Buu. Buu kills just about everyone on Earth. Supreme Kai sacrifices himself to bring back Goku. Baba brings Vegeta back to Earth. Buu destroys Earth. Porunga restores Earth. Vegeta's life is restored when all the good people that died are wished back to life. Goku destroys Buu with a Super Genki Dama.
Sept. 7, 774 Shenlong erases everyone's memories of Buu
May 7, 778 The 26th Tenkaichi Budoukai happens, ending with Hercule as the winner and Buu in second place.
779 Pan is born to Gohan and Videl
780 Vegeta and Bulma have their second child, Bra
May 7, 781 The 27th Tenkaichi Budoukai takes place. Hercule wins again and Buu again places second.
May 7, 784 The 28th Tenkaichi Budoukai begins. Goku leaves with Ubbu.
Dragonball GT
798 Pilaf wishes Goku into a child. The Black Star Dragonballs are scattered across the universe. Pan, Trunks and Goku set out in search of them. After nine months in space, Goku meets Bebi. Bebi is outnumbered, so he escapes to Earth, where he takes over nearly everyone and gains a great amount of power. He uses the Black Star Dragonballs to create Planet Tsufuru. Goku reaches Super Saiya-jin Level 4. He fights and kills Bebi. The entire Tsufuru race is destroyed. All the evil dragons appear. Vegeta uses Bulma's invention, the Brute Ray, to transform to SSJ 4. Goku leaves with Shenlong and is never seen again, except by Pan at a distant future Tenkaichi Budoukai.