21st century

The 21st century was, as Jack Harkness cryptically said, "when everything changes". (TW: Everything Changes) Among other things, this refers to the fact that the early 21st century was when the human race on Earth learned as a whole that alien life existed. (DW: The Christmas Invasion) Earlier chronicled invasions, including those of the Cybermen (DW: The Invasion) and the Slitheen family (DW: Aliens of London/World War Three), were believed by the general public to have been hoaxes, or otherwise covered up. (DW: Remembrance of the Daleks) Others, such as the Racnoss invasion (DW: The Runaway Bride), the disappearance of Royal Hope Hospital (DW: Smith and Jones) and the March of the Adipose (DW: Partners in Crime) were brief in duration, and most, such as the common incursions through the Rift (Torchwood) and conspicuous alien presences in Ealing (The Sarah Jane Adventures), though well-known enough to lead to the area being dubbed "the Ealing Triangle" (SJA: Goodbye, Sarah Jane Smith), were insignificant. However, as alien incursions became more frequent, it became harder to dismiss them, especially the worldwide invasions such as the Battle of Canary Wharf (DW: Doomsday) in 2007 and the Dalek Invasion of Earth (DW: The Stolen Earth) in 2009. The government publicly opened negotiations with at least two alien races, the Toclafane (DW: The Sound of Drums) in 2008 and the 456 (TW: Children of Earth) in 2009. Earth was a very tempting target for Invasion, even so that half of the species from Earth to the Verlixix Spiral Cluster has plans to invade. (SJA: Enemy of the Bane)
 * In a negated timeline, most people knew and accepted alien life by the 2050s (SJA: The Mad Woman in the Attic), although given events early in the century, odds are this fact remains in the restored timeline.

Specific events

 * In 2009, the Earth was relocated to the Medusa Cascade by the Daleks, causing untold damage around the world, which nonetheless unified in joy when the planet was returned to its proper place. (DW: The Stolen Earth / Journey's End) However, this was then unwritten from time by the Time Field.
 * This century, generally defined in Earth's calendar as covering the years 2001 to 2100, was a period of great technological advancement, but also great political, economic, and ecological turmoil. The T-Mat and the Gravitron were developed in the middle part of this century, with the first excursions outside the Solar System occurring towards the century's end.
 * Space travel increases dramatically. Many more missions are undertaken by NASA, the Russian Federal Space Agency, and European Space Agency (where all-German teams achieve ten lunar missions by 2050); the British Rocket Group is briefly revived in 2006; and Australia, Spain, and (possibly) the Philippines all get involved in space travel.
 * For forty years after 2019, Earth suffered through climate change, ozone degradation, the "Great Cataclysm", and "the oil apocalypse". In 2059, it was believed mankind came close to extinction. (DW: The Waters of Mars, K9TV: Aeolian)
 * The Thousand Day War between Earth and Mars was fought during this century. (NA: Transit)
 * Earth's first off-world colony, Bowie Base One, was both established and lost in the late 2050s. (DW: The Waters of Mars) It would later establish a permanent base. (BFBS: Missing Adventures: Home)
 * The United States falls during this century. (NA: Christmas on a Rational Planet)

Specific years
The following 21st century years were known to have had some significance to the broader course of history: