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A floating timeline (also known as a sliding timescale) is a device used in fiction, particularly in comics and animation, to explain why characters age little or not at all over a period of time — despite real-world markers like notable events, people and technology appearing in the works and correlating with the real world. A floating timeline is a subtle form of retroactive continuity. This is seen most clearly in the case of comic book characters who debuted as teens in the 1940s or the 1960s but who are still relatively young in current comics. Events from the characters' pasts are alluded to, but they are changed from having taken place years ago to having taken place more recently.
For an example taken from animation, in The Simpsons, episode "I Married Marge", which was broadcast in 1991, Homer and Marge are shown in flashback to have conceived their son Bart in 1980, after watching The Empire Strikes Back in the cinema. In an episode that was broadcast in 2008, "That '90s Show", however, another flashback shows Homer and Marge in an earlier stage of their relationship, but sets those scenes in the early 1990s grunge music era. In both sets of present-day scenes, the characters are shown to be the same age - for example, Bart is still 10 years old in 1991 and 2008.
For comic book examples, any dates given within the comic are not relative to the publishing date of the comic (i.e. "10 years ago" means ten years prior to the story being read by the reader). This device enables publishing companies to continue to use their characters for as long as they wish without changing them significantly.
A floating timeline is usually abstracted from that of actual historical events, but may contain subtle references to the real world timelines. There may also be attempts made to maintain certain characters' historic timelines if it is felt to be essential to the character's personality, while allowing the rest of the world's timelines to continue to develop. For example, in the 2000s comic book miniseries The Punisher and its subsequent continuation, the title character is shown to be a Vietnam War veteran, just as he was in his earliest appearances in the Marvel comics of the 1970s. However, the stories place him in contemporary New York, where he meets fellow Marvel characters such as Spider-Man, who is barely much older than he was when introduced in 1962. Different characters can also age at different rates within the same storyline. A good example of this is seen in The X-Men comics. In the 1960s when The X-Men were first introduced, the 3 characters Cyclops, Beast and Iceman all appeared to be (more or less) the same age, as teenagers. More recently however, Beast has typically been depicted as middle-aged, Cyclops in his early-mid 30s, and Iceman still in his early 20s. Meanwhile, some additional X-Men characters who debuted later appear to not have aged at all, particularly Storm, Gambit and Rogue.
An example taken from novels is the case of mystery writer Rex Stout, who created a floating timeline for master detective Nero Wolfe and other principal characters in the corpus, while the stories take place contemporaneously with their writing and depict a changing landscape and society. Nero Wolfe's age is 56. "Those stories have ignored time for thirty-nine years," Stout told his authorized biographer John McAleer. "Any reader who can't or won't do the same should skip them. I didn't age the characters because I didn't want to. That would have made it cumbersome and would seem to have centered attention on the characters rather than the stories."  In the early novels, Wolfe dated himself somewhat by discussing his life before World War I and his combat service in that war, but in later stories he was less explicit about his past.
- McAleer, John, Rex Stout: A Biography (1977, Little, Brown and Company; ISBN 0-316-55340-9), p. 383; and McAleer, John, Royal Decree (1983, Pontes Press, Ashton, MD), p. 49