Originally the 30th century, the 31st century is the time of origin of the Legion of Super-heroes. It was an usual time travel destiny for Superboy in the Silver Age, until it became the stable background for the Legion own adventures.
Despite the multiple reboots and revisions, the future universe of the Legion of Super-Heroes keep constant as the most possible future time from Earth-0, the mainstream reality
It served as background for Superboy adventures in the future where he befriend the teens from tomorrow. The Legion adventures where chronicled after the departure of Superboy. It would be during this time than the Legion background would grew more. The 30th Century is primarily identified as the height of the galactic federation known as the United Planets. The United Planets was a coalition of worlds that encompassed hundreds of planets within the Milky Way Galaxy and even a few outside the known galaxy, under the protection of the Science Police against threats as the Dominion and the Khunds. One curious element it was the absence of Green Lanterns, despite the organization still was in activity, as it was show with the existence of Universo, a former Green Lantern and Legion enemy. Other heroes than would rise during this period would be the Legion of Substitute Heroes, the Heroes of Lallor, and the Wanderers. Native from this time also are icons institutions like the Time Institute and the Legion Academy. For a brief time, it was also the era where Barry Allen went into retiring, before his demise in Crisis on the Infinite earths.
After the crisis, the 30th century was kept mostly the same. However the Legion discovered historic contradictions explained by the machinations of the Time Trapper which included the creation of a pocket universe with their own Superboy . This would stay the same until the Magic Wars, were all magic was released in a technological driven universe, making everything fail. When the magic was contained and the magic vanished, the United Planets suffer a wound from which wouldn't recover in many years.
After the magic wars, United Planets (UP) economy collapsed and the Dominion took control of earth government, controlling them from the shadows. The 30th century was now a more dangerous place, and the before friendly universe was now a hostile and repressive place to live. The new continuity changes were this time explained with the intervention of the villain Glorith who made changes in the time-stream to keep the Legion in existence and prevent the rise of Mordru, a magical Legion enemy, as absolute master of that era, including replacing Superboy with Valor (Lar-Gand aka Mon-El) as the Legion original inspiration.
After Zero Hour, the Legion era was rebooted. There was in this era when the Legion went from 30th century to 31st century. The origin was restarted and old stories were reinterpreted. In the new universe, the United Planet is a recently born organization and Brande industries had made the universe a more connected place thanks to the portals built by his company. The Legion now it was a UP sponsored organization. The Legion later would operate from an artificial planetoid called Legion World.
This version of the 30th/31st was later retconned out of existence and would reappear in Infinite Crisis as Earth-247. It would later return in Legion of 3 Worlds. The destiny of the Legion of this reality and their time after Flashpoint are unknown, but the 31st century of Earth-22 is a close erzats.
Started in the special Legion/Teen Titans, the threeboot setting for the Legion was a colder utopia were the Legion became the voice of youth rebellion. The future society was now very different. It is a time of stagnation and conformity, where physical contact with other people is considered the height of rudeness, and even people in the same room would use electronic means to converse rather than talk to each other face to face. Children are bio-tagged so their parents can track them anywhere, and are taught obedience and propriety. This reality would be revealed as the earth-prime 31st century in Legion of three worlds. If this had been changed after Flashpoint is unknown.
The original continuity was rescued by Geoff Johns, where a twist were given: an extreme xenophobia was grow among earth people and the planet had seceded from the United Planets and put aliens in ghettos. Starting from a unclear time previous to the magic wars, the Legion was now an outlawed team and fight against the Earthgov oppressionand hunted by the Justice League of Earth. Until the return of Superman, who revealed them his aliens origins, the anti-alien paranoia would grow becoming an element of the new background. Reintegrated to the UP, Earth would ban the anti aliens laws, but now the work of the Legion was harder to do. Specially with the Justice League of Earth operating against them.
Post-Flashpoint, the Legion era didn't suffer any major change and the continuity from the retro-boot With their origins retold in Legion: Secret origin were the intervention of the Time Trapper stopped their first encounter with Superman. Also, in the new Superman continuity, the 31st century has seen show Legion has worked with him. What other changes had suffered the 31st century now or how deep is still to soon to know.
Their last appearance is when the Legion traveled to the present to stop the existence of Infinitus, who was destroying the future. This Legion however showed the presence of several members from different timelines.
The status of the 31st century after the events of DC Rebirth is inconclusive. The era has been not showed, but the only clue is than Saturn Girl had traveled to the present day with unknown intentions. After being jailed in Arkham, she escaped and is helping Rorschach and Johnny Thunder in their own quests.
Alternate 31st Century
Because of the several continuity changes there are also yet other alternate universe 31st century than stand apart from mainstream canon 31st century:
- Superboy's Legion: In this elseworlds tale, Superboy is discovered in the 31st century and raised as a native of that time. Along with Cosmic Boy and Saturn Girl they found the legion and they must battle an evil than it was plotting his schemes since the 21st century.
- Legionnaires Annual 1: "Castles in the Air" shows a Legion and a 31st century more inspired in the Arthurian legends.
- Kingdom Come: Designated later as Earth 22, the 31st century of the Kingdom Come universe have a Legion similar to the Batch SW6/Earth-247, representatives of the Superman dynasty (Superboy and Supergirl in Legion uniform, who could be or not the originals who take the 31st century as home) and an elderly Superman.
- Mordruverse: A brief universe born after Lar Gand fought the Time Trapper and stopped him from create the conditions for the origin of the Legion. It was a 31st century where Mordru was the supreme master of the universe. However elements of resistance worked into erase this alternate timeline. From this point onward, the Glorithverse would born.
- Smallville season 11: a follow up to the Smallville series, here Supergirl and the Legion of Superheroes must deal with a 31st century where a city of kryptonians exists near earth.
- Justice League 3000-3001: an alternate future were new versions of the Justice League members has been recreated to oppose to the power of the Five. An alternate version of the Legion (composed by teenagers) existed in this universe but they were killed by Starro. A second Legion has appeared since then under the thrall of Lady Styx.
31st Century in other media
The 31st century has served as background for several stories in other media:
Superman the Animated Series:
In the chapter "New Kids on Town", a young Clark Kent is saved by time traveler legionnaires against a Brainiac decided to kill him before he became Superman.
Justice League Unlimited:
In the episode "Far from Home", Legionnaires Brainiac 5 and Bouncing Boy recruit the help of 21st century Justice Leaguers Green Lantern (John Stewart), Green Arrow and Supergirl to help them to stop the Fatal Five. The action is set in the future world of the 31st century.
Superman and the Legion of Superheroes:
Here the 31st century is the background of the adventures of the Legion and a young Superman who still can't fly and must learn how to fight along with other super-powered teens.
In the episode of Smallville entitled "Legion", Clark Kent could see briefly a glimpse from the future earth of the 31st century and only the city landscape silhouette.
In the third season of Supergirl, the episode Legion of Superheroes, members of this group appeared explaining the dangers than the presence of Reign means for their era.