A note about this version of the Legion Chronology:

This chronology is edited assuming that the DC universe is unified, and Superboy is from the Time Trapper's "Pocket Universe."

Legion Time Comments Issue #
BEFORE THE 20TH CENTURY
  At some unknown point in pre-history, the Time Trapper steals a single instant-long "slice" of the real universe, eliminates all inhabited worlds except Earth and Krypton, and later prevents the creation of all superheroes except for Superboy. (Action #591, 8.87), (Adventures of Superman 444, 9.88)
Four billion years ago On the planet Oa, Krona's forbidden experiment to witness the birth of creation causes the universe to split into the multiverse and unleashes a wave of evil into the cosmos. Krona is banished for his crime. His fellow Oans become the Guardians of the Universe. NOTES: This is the first chronological appearance of the Guardians of the Universe, whose first appearance in print was in Green Lantern #1 (July 1960). (Green Lantern #40, 10.65)
  A faction of the Guardians, dissatisfied with their colleagues' tactics in the war against evil, splits off to become the Controllers. NOTES: The Controllers first appeared in Adventure #357 (June 1967). Their connection to the Guardians was revealed in Crisis. (Crisis #7, 10.85)
Three billion years ago The Guardians devise a corps of warrior androids dubbed Manhunters. After the Manhunters rebel, they are disarmed and exiled from Oa. Over the coming millennia, the Manhunters secretly conspire against their former masters. (Justice League of America #141, 4.77), (Secret Origins #22, 1.88)
  The Guardians of Oa organize a new force to battle evil throughout the universe: the Green Lantern Corps (1st chronological appearance). The first Green Lantern is Rori Dag of the planet Rojira. NOTES: The Green Lantern Corps first appeared in Green Lantern v.2 #11 (Mar. 1962) (Green Lantern #67, 3.69)
c. One million B.C. Imsk, a planet around the star Irulan and the eventual homeworld of Salu Digby (Shrinking Violet), is colonized by the Krill. Eleven years after the colonists' arrival, they discover that the planet itself shrinks for a period of 90 days every ten years. To enable them to survive the shrinkage, an Imskian scientist finds a way to use the energies of radioactive Imskian rock to let all Imskians change their size at will. The ability is passed on to subsequent generations, but all Imskians must be exposed to the radiation once per year or they will die. (Superboy & the Legion #234, 12.77)
c. 12000 B.C. Traditional date for the beginning of Kryptonian civilization. NOTES: Krypton was destroyed its year 10,000; 18 Kryptonian years was equal to 25 Earth years. These dates apply only to the Krypton of the Pocket Universe. The civilization of Krypton began more than 500,000 years ago. (Action #223, 12.56)
c. 5000 B.C. The native humanoids of the planet Rimbor (eventual homeworld of Jo Nah, Ultra Boy) become extinct. (Legion v.4 #15, 2.91)
c. 2000 B.C. A time-lost Jo Nah (Ultra Boy) falls in love with a woman called Asherya, who is actually a Lord of Chaos. Nabu the Wise (Dr. Fate) sends him back to the 30th century. Legion v.4 #19 (June 1991)
1243 B.C. Searching for Universo, Shrinking Violet visits Gizeh (Giza), Egypt. Adventure #349 (Oct. 1966)
c. 209 B.C. The civilization of the planet Durla is nearly wiped out by a nuclear conflict called the "Six-Minute War." The inhabitants survive only through their shape-changing powers. Their original form is unknown. NOTES: The date was established by Legion v.2 Annual #2 (1983). (Legion v.2 #301, 7.83)
c. 500 A.D. The Guardians shrink the warrior giants called Djinn to tiny size and imprison them in bottles of green energy. (Legion v.2 #267, 9.80)
693 A.D. Colossal Boy tracks a time-traveling Universo to 7th century England, where, unable to control his growth power, he is attacked by medieval warriors who believe him to be a threatening giant. Adventure #349 (Oct. 1966)
1280 A.D. Pursuing Universo, Brainiac 5 travels to Shang-Tu, China, where he encounters the legendary Kublai Khan. Adventure #349 (Oct. 1966)
1300 A.D. In pursuit of Universo, Chameleon Boy appears in the Peruvian city of Cuzco, capitol of the Inca empire, where he is nearly slain by the jealous Inca god-king. Adventure #349 (Oct. 1966)
c. 1600 A.D. The island of Marzal is colonized by escaped slaves. The island fades into another dimension to return for 30-year periods every 200 years. (Legion v.2 #265, 7.80)
1812 A.D. A time-traveling Saturn Girl visits Paris, where she encounters Napoleon and is nearly slain by a French aristocrat under the hypnotic control of Universo. Adventure #349 (Oct. 1966)
THE SILVER AGE 20th CENTURY
Lar Gand is born on Daxam to Kel and Marisa Gand. NOTES: The year of Lar Gand's birth in any timeline has never been established, although the 2995 Sourcebook gave his birthday as March 27. *
35 Eorx, 9998 (Kryptonian calendar)
February 29 (Earth date)
On the Krypton of the Pocket Universe, Kal-El, the son of Jor-El and Lara Lor-Van, is born in Kryptonopolis, capitol of the planet Krypton. Notes: In the mainstream universe Kal-El was conceived in vitro on Krypton, but was not technically born until he landed on Earth. Superman's home planet was not named until the first episode of the Superman newspaper strip in January 1939. His Earth birthdate was established in World's Finest #235 (Jan. 1976). World of Krypton #2 (Aug. 1979)
3 Ullhah, 9999 (Kryptonian calendar) On the Krypton of the Pocket Universe, Jor-El discovers the Phantom Zone, which Krypton's Science Council decides to use as a prison for Kryptonian criminals. Its inmates later include General Zod, Faora Hu-Ul, and Quex-Ul. NOTES: The date was given in World of Krypton #2 (Aug. 1979). As first established in Adventure #323 (Aug. 1964), the Phantom Zone is apparently the same dimension as, or contiguous to, the Bgztl Buffer Region. Kryptonians of the mainstream DC universe did not use the Zone as a prison. (Adventure #283, 4.61)
54 Belyuth, 10,000 (Kryptonian calendar) Kal-El's puppy Krypto is lost in space. NOTES: The date was given in World of Krypton #3 (Sept. 1979). (Adventure #210, 5.51)
34 Ogtal, 10,000 (Kryptonian calendar) Daxamite explorer Lar Gand (Mon-El) is diverted to the Pocket Universe where he lands on Krypton and meets Jor-El and Lara. NOTES: The date was given in World of Krypton #3 (Sept. 1979). Mon-El's real name was not revealed until the reprint of this story in Superboy #129 (May 1966). (Superboy v.1 #89, 6.61)
39 Ogtal, 10,000 (Kryptonian calendar) A small group of Legionnaires visits Krypton on the day of its destruction. NOTES: The date of Krypton's destruction was given in World of Krypton #3 (Sept. 1979). Legion v.2 #255 (Sept. 1979)
  Moments before Krypton's destruction, two-year-old Kal-El is launched into space aboard a tiny starship, bound for Earth. (Action #1, 6.38)
  In the Pocket Universe, Kal-El lands in Smallville, where he immediately gains super powers under Earth's yellow sun. He is adopted by Jonathan and Martha Kent and given the name Clark Kent. In the mainstream DC universe Clark Kent's powers develop gradually and do not fully manifest until he is almost 18 years old. NOTES: Superman's foster parents first appeared in Superman #1 (Sum.39). Action #1 (June 1938) said only that he was found by "a passing motorist." Jonathan Kent was first named in Adventure #149 (Feb. 1950), Martha Kent in Superboy #12 (Feb. 1951). The origin of Clark Kent was recounted in Man of Steel #1 (June 1986). (Superman v.1 #1, Sum.39)
  The radioactive boulders that renew Imskians' shrinking powers are consumed by a race of space dragons. Afterwards, the people of Imsk must periodically hunt the dragons and remove some of their scales, which emit the radiation necessary for the Imskians' survival. (Superboy & the Legion #234, 12.77)
  In the Pocket and mainstream universes, Jonathan and Martha Kent fashion their adopted son's Kryptonian blankets into an invulnerable costume. Same event in Pocket Universe. Notes: In the mainstream universe, as first mentioned in Man of Steel #1 (June 1986), Superman's costume is partially protected by his own invulnerability, but it is not indestructible, nor is it made of fabric from Krypton. (Superboy v.1 #78, 1.60)
  In the Pocket Universe, at age 3, Clark becomes Superbaby. (Superboy v.1 #26, 6-7.53)
  At age 6, Clark enters public school. Superboy v.1 #75 (Sept. 1959)
  In the Pocket Universe, 8-year-old Kal-El goes into action as Superboy. In the mainstream universe Superman did not begin his costumed career until adulthood. More Fun #101 (1-2.45)
  The infant Iris Russell appears from the 30th century and is adopted by the West family. NOTES: Iris West and Barry Allen (Flash II) are the parents of Don and Dawn Allen, the 30th Century "Tornado Twins." Iris first appeared in Showcase Comics #4 (Oct. 1956). Don and Dawn Allen first appeared in Adventure Comics #373 (Oct. 1968). (Flash v.1 #350, 10.85)
  In the Pocket Universe, Superboy meets Lana Lang. In the mainstream universe Lana Lang and Clark Kent were childhood friends, but Clark had no career as Superboy. Superboy v.1 #10 (Oct. 1950)
  Clark turns 10. Adventure #103 (Apr. 1946)
  12-year-old Clark meets the Kryptonian robot teacher. Note: DC Super-Stars #12 (Feb. 1977) states his age for this event. Adventure #240 (Sept. 1957)
  In the Pocket Universe, Krypto arrives on Earth and is reunited with his master. In the mainstream universe Krypto is a normal dog adopted as a puppy by Superman's friend Bibbo Bibowski. He was first seen in Adventures of Superman #501 (L6.93). Adventure #210 (May 1955)
  14-year-old Clark meets alien Qor Sulor. Note: The story states it had been twelve years since his arrival on Earth (age 2). Superboy v.1 #137 (Apr. 1967)
1st app. The three founding members of the Legion of Super-Heroes (Cosmic Boy, Lightning Lad, and Saturn Girl) meet Superboy. He visits their clubhouse in 30th century Smallville, where he meets their other members, including Brainiac 5, and joins the Legion. NOTES: Other Legion members are shown in shadow, suggesting that although this was the Legion's first appearance, they'd formed some time previously. Brainiac 5 clearly appears in the background of one panel, although he is not named. This portion of the chronology assumes that these early stories were told in the order in which Superboy experienced them. This is different than the order in which the Legionnaires experienced them (for these and other details, scroll down to the events of the 30th Century). This story describes the Legion as existing in the 30th century, although some subsequent stories placed it in the 21st century instead. Most Legion stories placed the Legion's adventures exactly 1,000 years in the future. Adventure #247 (Apr. 1958)
In the Pocket Universe, Superboy has his first encounter with Red Kryptonite, an isotope of Kryptonite altered by its passage through a cosmic cloud, causing it to have weird, unpredictable (though fortunately temporary) effects on Kryptonians. NOTES: In its first appearance, Red-K was described as simply being "100 times" more potent than normal Green Kryptonite. Its weird effects on Superman were first demonstrated in its second appearance in Adventure Comics #255 (Dec. 1958). Action Comics #591 (Aug. 1987) established that Superman was immune to all of the varieties of Kryptonite of the Pocket Universe. A sample of Red Kryptonite was created by Mr. Mxyzptlk in Superman v.2 #49 (Nov. 1990), but its effectiveness was solely a product of Mxyzptlk's magical powers; it was not true Kryptonite and probably was not even radioactive. A different type of Red Kryptonite was created by Batman as a contingency plan against Superman, as seen in JLA #44-45 (8-9.00). Adventure #252 (Sept. 1958)
2nd app. The Legion tries to stop Superboy from going on a rampage, unaware that he is actually on a secret mission for the President. No similar events. NOTES: The Legionnaires' costumes are slightly different in this story than in their first appearance. They still use rocket belts rather than flight belts, but their names are no longer printed on their chests. Starting in this story, Saturn Girl's costume is colored pink and white rather than green and yellow. Adventure #267 (Dec. 1959)
The time-traveling Karate Kid meets Superboy and a group of young Legionnaires, who help him battle the Lord of Time and Major Disaster. The Lord of Time blasts Karate Kid forward in time and erases the Legionnaires' memories of these events. NOTES: The Lord of Time first appeared in Justice League of America #10 (Mar. 1962), Major Disaster in Green Lantern v.2 #43 (Mar. 1966). Karate Kid #12-13 (1-4.78)
Clark Kent meets Pete Ross. Superboy v.1 #86 (Jan. 1961)
5th app. The Pocket Universe Lana Lang visits the 30th century with Superboy, where she uses new Legionnaire Star Boy in an effort to make Superboy jealous. Adventure #282 (Mar. 1961)
7th app. Lar Gand lands on the Pocket Universe Earth, suffering from amnesia, and meets Superboy, who dubs him Mon-El. He briefly moves into the Kent home, adopting the identity of traveling salesman "Bob Cobb." He regains his memory after suffering from lead poisoning. To save his life, Superboy is forced to send him to the Phantom Zone. Superboy v.1 #89 (June 1961)
Pete Ross learns that Clark Kent is secretly Superboy. He does not inform his friend of his discovery. NOTES: Clark Kent did not find out that Pete learned his secret identity until they were adults, as shown in DC Comics Presents #13 (Sept. 1979). The Pete Ross of the mainstream universe is not aware of Superman's true identity. Superboy v.1 #90 (July 1961)
9th app. A 30th century criminal posing as Sun Boy fools Superboy into reassembling a Cyclops robot buried in Smallville by the Legion. Adventure #290 (Nov. 1961)
Chameleon Boy visits Superboy to write an article for the Legion newspaper. No similar events. Superboy v.1 #93 (Dec. 1961)
As his test for Legion membership, Ultra Boy and his friend Marla Latham visit the 20th century, where Ultra Boy, using the name "Gary Crane," attempts to uncover Superboy's secret identity. Later, Ultra Boy and Marla make Pete Ross an honorary Legionnaire as a reward for being Superboy's loyal friend. NOTES: This story contains the first telling of Ultra Boy's origin and the first mention of his home planet, Rimbor. Superboy v.1 #98 (July 1962)
In the Pocket Universe, Superboy encounters Gold Kryptonite, an isotope of Kryptonite with the power to strip a Kryptonian of his or her super-powers forever. Notes: Action Comics #591 (Aug. 1987) established that Superman was immune to all of the varieties of Kryptonite of the Pocket Universe. Adventure #299 (Aug. 1962)
While Superboy is visiting the Legion, Pete Ross has problems with a malfunctioning Superboy robot, leading Ultra Boy to travel back in time to help him. Superboy v.1 #100 (Oct. 1962)
Brainiac 5 helps Superboy capture Blackie Burke. Adventure #309 (June 1963)
Colossal Boy helps Superboy protect his secret identity. Adventure #315 (Dec. 1963)
Pete Ross visits the Legion in the 30th century and participates in a contest to determine the new Legion leader. Adventure #323 (Aug. 1964)
Lana Lang becomes Insect Queen. Superboy v.1 #124 (Oct. 1965)
Kid Psycho asks Superboy's help in joining the Legion. He eventually becomes a reserve member. Superboy v.1 #125 (Dec. 1965)
Lana Lang again visits the 30th century, where, as Insect Queen, she becomes an reserve Legionnaire. Adventure #355 (Apr. 1967)
Polar Boy and his parents help Superboy battle a race of flame creatures. Superboy v.1 #148 (June 1968)
Duo Damsel, Mon-El, Shadow Lass, and Superboy take refuge from Mordru in Superboy's era, where the 30th century Legionnaires poses as distant relatives of various Smallville residents. Adventure #369 (June 1968)
The Legionnaires deliberately remove their own memories of their true identities in a futile attempt to escape Mordru. Despite their efforts, Mordru captures them, along with Pete Ross and Lana Lang, and puts them on trial. The Legionnaires ultimately escape with the help of Dream Girl and the White Witch. Adventure #370 (July 1968)
  Superboy battles Mordru in the 20th century and imprisons him by burying him. He tosses Mordru's magical Blood Crystals into deep space, where they are later discovered by the Legion. Chameleon Boy, afflicted by the Crystals with an irrational hatred of Superboy, travels back in time to attempt to destroy the Boy of Steel. Superboy v.1 #188 (July 1972)
The Fatal Five attempts to eliminate the Legion from history by killing Superboy in the 20th century. Superboy v.1 #198 (Oct. 1973)
Lana Lang travels to the 30th century for an adventure as Insect Queen. Superboy v.1 #205 (Nov./Dec. 1974)
Superboy encounters the Legion's clone duplicates of the deceased Invisible Kid and Ferro Lad. Superboy v.1 #206 (1-2.75)
Superboy returns to his own era under a hypnotic compulsion from Saturn Girl not to return to the 30th century. Legion v.2 #259 (Jan. 1980)
Superboy celebrates his 16th birthday. New Adventures of Superboy #1 (Jan. 1980)
Ultra Boy is thrown back to the 20th century and trapped in the Bgztl Buffer Region. His mind is transferred into Superboy's body. Afflicted with amnesia, he adopts a new costumed identity as Reflecto and returns to the 30th century, where he re-applies for Legion membership. NOTES: These events follow Ultra Boy's apparent death in Legion v.2 #275 (May 1981). (Legion v.2 #282) (Dec. 1981)
The Legion travels back to the 20th century to learn why Superboy was costumed as Reflecto. NOTES: This story continues from Legion v.2 #279 (Sept. 1981). Legion v.2 #280 (Oct. 1981)
The Legionnaires and Superboy battle the Molecule Master and are arrested by the U.S. Army for interfering with an atomic bomb test. Phantom Girl, Dawnstar, and Superboy escape to 20th century Bgztl. Legion v.2 #281 (Nov. 1981)
Phantom Girl, Dawnstar, and Superboy find Ultra Boy in the Bgztl Buffer Region and learn how he (in Superboy's body) became Reflecto. Superboy decides to resume active membership in the Legion. Legion v.2 #282 (Dec. 1981)
THE POCKET UNIVERSE SAGA
  The Legion encounters severe time storms while traveling to Superboy's time. On their way back to the 30th century they briefly appear in the real universe in the year 2050. NOTES: The Legionnaires' brief appearance in the 21st century is seen concurrently in Hex #10 (June 1986). Legion v.3 #23 (June 1986)
  Superboy finds Smallville threatened by waves of anti-matter. The Time Trapper offers to save Smallville for a price: Superboy must help him eliminate the Legion. NOTES: These events were shown in flashback in Legion v.3 #38. (Legion v.3 #38) (Sept. 1987)
  A group of Legionnaires is shunted back to Superboy's time by the Time Trapper. While Brainiac 5, Blok, Invisible Kid, and Sun Boy attempt to return to the 30th century, the others are attacked by Superboy. Legion v.3 #37 (Aug. 1987)
  Blok, Brainiac 5, Invisible Kid, Sun Boy, and the real Superman appear in Smallville, where Superman clashes with Superboy and Krypto. Krypto loses his powers trying to expose Superman to Gold Kryptonite, which has no effect. Superboy confesses that he is being blackmailed by the Time Trapper. NOTES: This story first revealed how the Trapper created the Pocket Universe. Action #591 (Aug. 1987)
Superboy defies the Time Trapper and sacrifices himself to save the Pocket Universe. He carries the Legionnaires back to the 30th century and dies in Mon-El's arms. The residents of the Pocket Universe are unaware of his fate. Legion v.3 #38 (Sept. 1987)
In the Pocket Universe, Lex Luthor arrives in Smallville and is tricked into releasing the Kryptonian criminals General Zod, Quex-Ul, and Zaora from the Phantom Zone. They begin a destructive rampage. The Pocket Universe's Lana Lang is among those killed. NOTES: This is the first appearance of the Pocket Universe Phantom Zone criminals. General Zod first historical appearance was Adventure Comics #283 (Apr. 1961), Quex-Ul in Superman v.1 #157 (Nov. 1962), and Faora Hu-Ul, on whom Zaora is modeled, in Action #471 (May 1977). (Adventures of Superman #444, 9.88)
Superman's Era
Clark Kent makes his first public super-powered outing in Metropolis, saving the crippled space plane Constitution. He meets Lois Lane, who dubs him "Superman." He briefly returns to Smallville, where his foster parents reveal his extraterrestrial origins and he is accidentally exposed for the first time to Kryptonite (although he is unaware of it). Fashioning a costume for himself, he returns to Metropolis to begin his superhero career as Superman. His presence brings about the rise of a new heroic age. Man of Steel #1 (June 1986)
Darkseid, the master of Apokolips, begins his search for the Anti-Life Equation in the minds of the people of Earth. Jimmy Olsen #134 (Dec. 1970)

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