The World History of the Code Geass Universe differs markedly from our own, mostly in the outcome of major battles and war, but also in the appearance of materials like Sakuradite and immortals like C.C..

Calendars

The Imperial Calendar refers to the date and setting Code Geass uses in the English version, in which years are labeled as "a.t.b" or "Ascension Throne Britannia". It is set to the date the first Celtic super-king, Alwin I, was elected by the united British tribes to battle Rome, becoming the first member of the modern Britannian imperial family. This calendar is used by the Holy Empire of Britannia. It functions similarly to the standard Gregorian Calendar (B.C.E./C.E. or alternatively B.C./A.D.), but starts half a century earlier.

As referenced in Code Geass: Akito the Exiled, the Europia United shown is using the French Republican Calendar. The first entry in Leila Malcal's journal in the first episode of Code Geass: Akito the Exiled is dated as "Revolutionary Calendar, year 228, Floréal 18th" (a.t.b. May 7, 2017), and in the second episode "Revolutionary Calendar, year 228, Prairial 7th" (a.t.b. May 26th, 2017).

As the events of the first episode of Akito the Exiled occur in the year 2017 a.t.b., it is safe to assume that the Revolutionary Calendar starts with the Storming of the Bastille, which occurred at July 14, 1789 (2017 - 228 = 1789). It revealed as set on same year in our world. But revealed a.t.b. and A.D. have differences officially.^[1]

Following the death of Emperor Lelouch at the hands of Zero, a new calendar system is adopted from the following year on, referred to as Light Sun.

Timeline

• Pre-a.t.b. (58 - 51 B.C.)
  • Military expedition by Julius Caesar (100 - 44 B.C.), invades Britain twice in 55 and 54 B.C. The Gallic expedition a record of fighting at the time of "Gallic War" is all that is himself wrote. The results of the expedition over several times, almost the entire area of Gaul was incorporated into Roman territory, further increasing his fame.
• First year of a.t.b. / First year of A.D.
  • Alwin I meets with strong resistance from the local tribes, who repulse Roman Empire and he summarily becomes the first member of the Britannian Imperial Family. He won the independence from the Roman Empire of Augustus reign (27 B.C. - 14 A.D.), and the first year it was crowned as Celtic tribal leaders. The calendar was established by Ricardo von Britannia I.
• 14th - 15th Century a.t.b.
  • Sakuradite (known as the "Philosopher's Stone" at the time) is discovered near Stonehenge. The scarcity of sakuradite impedes the research to turn it into a viable energy source. However, in his travels, Marco Polo journeys further into the East, reaching Japan and discovers the nation's large deposit of sakuradite.
  • C.C. is born.
• 17th Century a.t.b.
  • Elizabeth I, who remained single throughout her life, bears a son, Henry IX. The potential fathers — Sir Robert Dudley, 1st Earl of Leicester; Sir Robert Devereux, 2nd Earl of Essex; and Sir Carl, Duke of Britannia — gain influence and power with this knowledge.
  • Henry IX ascends to the throne after the death of his mother in 1603 a.t.b., starting the Golden Age of the Tudor Dynasty.
• 1760s - 70s a.t.b.
  • The American Revolution (also known as Washington's Rebellion) occurs. The Duke of Britannia bribes Benjamin Franklin with promises of titles and territories in the colonies, who was charged with appealing to Louis XVI for assistance in the American colonies' war for independence.
  • There after, Benjamin Franklin is given the title of Earl. As a result, the Continental Army suffers a decisive defeat during the Siege of Yorktown with the death of George Washington, marking a severe blow to the American movement for Independence.
• July 14, 1789 a.t.b.
  • The storming of the Bastille occurs in France.
• Late 18th / Early 19th Century a.t.b.
  • The western world enters the Age of Revolution, with numerous national revolutions taking place, except in the British Isles, under the rule of King Henry X, who continues to hold absolute monarchy.
  • After French revolutionary Napoleon Bonaparte emerges, in 1805 a.t.b. he wins the Battle of Trafalgar, invades Great Britain, and occupies London. In 1807 a.t.b Queen Elizabeth III retreats to Edinburgh, where a revolutionary militia arrests her and forces to abdicate, ending the monarchy. This event becomes known as The Humiliation of Edinburgh. However, Sir Ricardo von Britannia, Duke of Britannia, and his friend and subordinate, Sir Richard Hector, Knight of One, bring Elizabeth III and her followers to the New World and establish a capital on the east coast of North America. The British Isles are conquered.
• 1813 a.t.b.
  • Elizabeth III nominates her lover, Sir Ricardo von Britannia, as her successor upon her death. She ends her reign for being "the Queen who loved throughout her stormy life". The Britannian calendar, Ascension Throne Britannia (a.t.b.), is established, with the origin year set to the ascendancy of the first Celtic king, though the calendar's months and days are adopted from the Gregorian Calendar. The accession of Ricardo as King is the transition from the United Kingdom of Great Britain to the Holy Britannian Empire.
• 1814-1821 a.t.b.
  • The revolutionaries stop Napoleon Bonaparte before he is able to become emperor, killing the hero of the revolution via guillotine and therefore saving the revolutionary republic from becoming what it had been fighting against. This leads to the formation of the Europia United.
• 1853 a.t.b.
  • Britannia finishes stabilizing its country and begins looking for foreign opportunities, especially in the Pacific. Commodore Matthew Perry crosses the Pacific with four black ships with outer rings that are operated with electric motors. They discover Japan, who have an isolationist policy. Japan realizes that they'd fallen behind the rest of the world.
• 1860s a.t.b
  • Britannia enters a succession crisis after the death of Emperor Aurelien. The presumed successor and First Prince Basilio is murdered by former noble Ganelon, who frames the Weinberg family. The Knight of One, Christoph Chamberlain, goes missing, causing the Knights of the Round to scatter. 
  • Two prominent factions arise, the Legitimate faction led by the House of El and the Purist faction, led by Lorenzo il Soresi.
  • Claire li Britannia is retrieved and escorted back from Japan by Alto Weinberg, Renya, and their friends. Claire, in place of Prince Basilio's five-year-old son, is backed by the Legitimate faction. Lorenzo is placed under Geass by Shiori to prioritize loyalty to the royal family rather than fulfilling his ambitions.  Claire li Britannia becomes empress.
• Late-19th, Early-20th Century a.t.b
  • 
    Trench warfare in C.C.'s flashbacks of World War I-like events
  • 
    C.C.'s memories of a war in the past, featuring early tanks and airplanes.

  1/2
  • A war featuring biplanes, tanks, and trenches is fought as seen in C.C.'s flashbacks in Episode 25 of R1, which was probably fought in the E.U. Also in the flashback, C.C. was shot by possibly a German soldier (considering the silhouette).
• 1944 a.t.b.
  • Japan loses a major war (described in 2010 a.t.b as having taken place 65 years earlier) embracing democracy as a result (Mamoru Iwasa, Code Geass Stage-0-Entrance Light Novel, p.120-121).
• 1955 a.t.b.
  • The 'Emblem of Blood' era. During this period, assassination and betrayals were frequented among the member of the Imperial Family that coveted the throne. At the same time, Princes V.V. and Charles zi Britannia are born under their mother, who herself is later assassinated. The reason behind this is unknown.
• September 1983 - August 1984 a.t.b.
  • 1st Prince Odysseus eu Britannia is born.
• September 1985 - August 1986 a.t.b.
  • 1st Princess Guinevere de Britannia is born.
• 1990 a.t.b.
  • 2nd Prince Schneizel el Britannia is born.
  • 2nd Princess Cornelia li Britannia is born.
• 1993 a.t.b.
  • 3rd Prince Clovis la Britannia is born.
• 1998 a.t.b.
  • The 97th Emperor of Britannia is overthrown, and Charles zi Britannia ascends to the Britannian throne. Charles also marries Marianne vi Britannia. With V.V. they plan to build a weapon to destroy the Gods.
• 1999 a.t.b.
  • Leila Malcal is born.
  • 11th Prince Lelouch vi Britannia is born.
• 2000 a.t.b.
  • Akito Hyuga is born.
  • Suzaku Kururugi, son of Japanese Prime Minister Genbu Kururugi and future Knight of Seven, Knight of Zero, and second Zero, is born.
  • 3rd Princess Euphemia li Britannia is born.
• 2002 a.t.b.
  • 5th Princess Carine ne Britannia is born.
  • Rolo Lamperouge, Nunnally's replacement is born.
• 2003 a.t.b.
  • Princess Nunnally vi Britannia is born.
  • Shin Hyuga Shaing kills all his family members. Akito Hyuga is the only member who survives.
• 2009 a.t.b.
  • Spring, 2009 a.t.b 
    • Marianne vi Britannia is killed by V.V. Her children, Lelouch vi Britannia and Nunnally vi Britannia, are sent to Japan as political hostages. The public is told they are studying abroad.
    • Britannia's Viceroy Government officially proposes a reform on a technological agreement concerning the bioelectronics field. Japanese officials agree that it is a move to sanction Japan. Prime Minister Kururugi immediately releases a statement of opposition and also states that he is considering presenting this case to an international court.
    • International human rights group NAR issues a statement condemning the policies the Holy Britannian Empire imposes on Numbers. The empire ignores the statement, to the critisisim of many around the world.
    • Britannian military forces conduct air strike on Hanoi, the capital of Vietnam, claiming they are performing limited attacks on guerilla military bases. According to a Japanese military official, based on the scar on the landscape, no effort was made to reduce collateral damage against ordinary citizens.
    • Britannia's Foreign Ministry claim Japan is supporting terrorist activities such as a bombing in Manila, capital of the Philippines.
    • Britannia fleet passes through territorial limit of Ishigaki Island. 
    • Britannian patrol craft fires warning shots at Japanese fishing boast off the coast of Alaska and preforms a forced search on suspicion of drug smuggling.
  • Summer, 2009 a.t.b
    • Japan takes advantage of hostilities between the Chinese Federation and Britannia to maintain its own neutrality. By manipulating the distribution of sakuradite, Japan creates a three-way standoff between the Chinese Federation, the EU, and Britannia. For a time, it enjoys economic prosperity.
    • After the Indochinese Peninsula was conquered by Britannia and named Area 10, Japan, which was originally neutral, decided to align its policy with Chinese Federation and E.U.'s politics and apply economic pressure upon Britannia -- an event referred to as the Oriental Incident. Both the Chinese Federation, the E.U. and their allies blockade the ports of Britannia and apply economic sanctions in an effort to come to negotiations.
    • Britannia reacts in extremis, both sides expand military and information gathering.
• 2010 a.t.b
  • The common assumption around the world in international relations at the time is that attacking Japan would spark a full-scale global war among all three superpowers. Charles zi Britannia blindsides the other factions by sending all Knights of the Round to Africa and India, and deploying his flagship, the Great Britannia, to the Indian Ocean. While all eyes turn away from the Pacific.
  • August 10, 2010 a.t.b
    • The Second Pacific War erupts; according to Lelouch, the war was all but decided within 24 hours, and by the time the Chinese Federation and the EU realized what had happened, it was too late. Japan is conquered in one month by Britannia. The end of the war signifies Japan as a formal colony of Britannia, renamed Area 11 and its citizens "Elevens."
• 2012 a.t.b.
  • Tianzi and Xingke exchange a pledge of permanent harmony.
• 2017 a.t.b. / Revolutionary Calendar, year 228
  • August 2017 a.t.b.
    • Viceroy and Third Prince Clovis la Britannia is killed by Zero.
    • Second Princess Cornelia li Britannia is appointed Viceroy of Area 11, naming Third Princess Euphemia li Britannia as sub-viceroy. Cornelia immediately begins her campaign to bring Zero to justice.
    • Zero forms the Order of the Black Knights. Its membership expands with each victory. All attempts to destroy the Black Knights end in failure.
  • September 2017 a.t.b.
    • Using her power, Euphemia li Britannia tries to create a Special Administrative Zone of Japan.^[2] It is "revealed" to be a plot to massacre the Elevens, and she is killed by Zero.
    • The Black Rebellion breaks out in the aftermath of the failed attempt to create the Special Administrative Zone. The Black Knights ignite riots across the country as they continue to push towards the Area 11 Viceroy Palace. The rebellion is eventually pushed back and crushed, with most of the Black Knights killed or captured. Area 11 is demoted to a correctional sub-area.
    • Second Princess Cornelia li Britannia goes missing during the Black Rebellion. Her position as Viceroy of Area 11 later is taken by Duke Calares.
• October 2017 a.t.b - April 2018 a.t.b / Revolutionary Year 228-229
  • Inspired by Zero, rebellion spreads across the empire. Glinda Knights and the Knights of the Round are dispatched to certain Areas to maintain control.
  • E.U. Army's 132nd Regiment encircled at Narva on the way to liberate Sankt Petersburg from Euro Britannia. W-0 Unit successfully breaks encirclement at the cost of all pilots lives except Akito Hyuga].
    • According to Leila's journal, the date of this operation is May 7, 2017 a.t.b / Floréal 18th, year 228​​​
  • Shin Hyuga Shaing murders Michele Manfredi and is accepted as the next Grand Master of the Holy Order of Michael.
  • W-0 and Holy Order of Michael clash in Slonim, Emperor Charles sends Julius Kingsley and Suzaku Kururugi to take command of all Eurasia operations.
  • Julius Kingsley enacts ARC Fleet plan to incite mass chaos in Europe while Euro Britannia invades. Grand Duke Augusta Henry Velaines] refuses, citing mass casualties among civilians. Kingsley has Velaines arrested for questioning his orders.
  • Shin Hyuga Shaing undertakes a coup d'etat of Euro Britannia. Julius Kingsley and Suzaku Kururugi are jailed, Grand Masters Gaudefroy du Villon and  Raymond du Saint-Gilles are beheaded, and Euro Britannia grants Shin emergancy powers in order to combat the E.U and Britannia.
  • Julius Kingsley and Suzaku Kururugi are released by Geass Order agents led by Rolo.
• 2018 a.t.b. / Revolution Year 229
  • May 2018 a.t.b.
    • The remaining members of the Black Knights incite riots in Babel Tower which kill Calares.
    • Within the Chinese Federation Consulate of Area 11, Zero reappears and redeclares the United States of Japan.
    • Europia United is losing to Britannia leading to more colonization of its countries to areas
    • The former Knight of Princess Cornelia, Gilbert G. P. Guilford, declares himself as new Viceroy. He then announces to execute Black Knights' members, although it's failed thanks to Zero's crafty strategy.
    • Princess Nunnally vi Britannia becomes Viceroy of Area 11, and attempts to reestablish the Special Administration Zone of Japan. Zero gives his support on the condition that he be exiled. Zero, along with over a million of his supporters dressed like him, are exiled from Area 11 and seek asylum on Penglai Island located within the political borders of the Chinese Federation. Because all of Zero's supporters were the participants in the zone, the attempt ends in failure.
    • Li Xingke and Zero disrupt the arranged political wedding between First Prince Odysseus eu Britannia and Empress Tianzi of the Chinese Federation.
    • The High Eunuchs are summarily executed for lèse majesté against the Empress. An alliance between the Black Knights and the Chinese Federation is forged.
  • June 2018 a.t.b.
    • After the fragmentation of the Chinese Federation, Second Prince Schneizel el Britannia prepares to annex many of its territories through a combination of diplomatic means, which results in failure.
    • Europia United is highly conquered
    • Ratification of United Federation of Nations is completed, with the Order of the Black Knights to serve as its chief military branch. Its first resolution is to take action against the occupying Britannian forces in Japan, which leads to a declaration of war between the U.F.N. and the Holy Britannian Empire.
    • The U.F.N. initiates the Second Battle of Tokyo to reclaim Area 11. The first use of F.L.E.I.J.A. by the Britannians results in massive damages to the concession area of the Tokyo Settlement.
    • Zero is reported to be killed during the Second Battle of Tokyo. However, there are rumors that this is falsified information for reasons unknown. In light of the alleged death of Zero, a truce is negotiated between U.F.N. and Britannia, with a peace treaty being drafted. The treaty results in the neutrality of Japan.
    • A coup d'etat is enacted by Second Prince Schneizel and the Knight of Seven, Suzaku Kururugi. Meanwhile, Lelouch vi Britannia instigates his own rebellion, forcing his followers to his will with the power of Geass.
    • 98th Emperor, Charles zi Britannia is killed by Lelouch, the former 11th Prince.
  • July 2018 a.t.b.
    • One month after the Second Battle of Tokyo, Lelouch vi Britannia coronates himself as the 99th Emperor of the Holy Britannian Empire and appoints the title of "Knight of Zero" to Suzaku Kururugi (heretofore the Knight of Seven). With his ascension come many changes in Britannian policies, including the destruction of the Imperial Mausoleum and the abolishment of privileges to nobles. This leads to many attempts of rebellion (one of which is led by the Knight of One, Bismarck Waldstein), each of them futile against the "Emperor of Justice".
    • Britannia attempts to join the U.F.N., and negotiation takes place in Japan, now a neutral zone. However, the leaders of the U.F.N. and the Black Knights attempt to convince Emperor Lelouch to lower Britannia's voting rights as a balance for the U.F.N.. The attempt fails, and in response Britannia launches an invasion against Japan, capturing the leadership of the U.F.N..
    • F.L.E.I.J.A. bomb is dropped on the Britannian capital of Pendragon from the Aerial Fortress Damocles, created by the Toromo institute, by Former 2nd Prince Schneizel resulting in the complete decimation of the capital.
    • The battle of Mount Fuji begins. Both sides suffer heavy losses, but eventually Emperor Lelouch is able to take control of Damocles and ends the battle, as well as the war between Britannia and the U.F.N. , with a display of power from the Damocles.
  • September 2018 a.t.b.
    • Two months after the battle, using U.F.N. as political tool for forcing E.U. to radicate, Emperor Lelouch vi Britannia declares himself world leader. During a supervision of executions of the Black Knights and U.F.N. leaders, Suzaku Kururugi masquerades as Zero and assassinates Lelouch to complete the Zero Requiem.
    • Nunnally vi Britannia begins to work in cooperation with the U.F.N. to achieve peace.

Story order

Canonical

1860s a.t.b.

• Code Geass: Renya of Darkness - 1860s a.t.b. (Around American Civil War; 1861-1865)

2017 a.t.b.

• Code Geass: Lelouch of the Rebellion - August 15 - September 9
  • Ghost Full moon in Stage 20, Halloween and Christmas in R2 photos
• Code Geass: Oz the Reflection - R1.5; Between first and R2. October 4 - November
• Code Geass: Akito the Exiled
  • First Episode: Late April and Early May
  • Second Episode: A month later, June
  • Third Episode: A month later, Mid-July and Late July
  • Fourth Episode: Early August, Mid and Late October
  • Final Episode: November and Mid-December, 3 months later and March

2018 a.t.b.

• Code Geass: Lelouch of the Rebellion R2 - May - September
• Code Geass: Oz the Reflection O2 - Same timeline as second season. Forget memories after 6 months from Oz First.
  • Episode 13 is set on Final Battle Tokyo II before.

2020 a.t.b.

• Code Geass: Lelouch of the Re;surrection
• Code Geass: Lelouch of the Re;surrection (Manga)

2022 a.t.b.

• Code Geass: Pure Almaria

2025 a.t.b.

• Code Geass: Rozé of the Recapture

Non Canonical

• Light Novels - August - December. Separated story from Original. Unofficial story.

1867 a.t.b.

• Code Geass: Tales of an Alternate Shogunate

2017 a.t.b.

• Code Geass: Nightmare of Nunnally
• Code Geass: Suzaku of the Counterattack

References