First officially referenced in 2008's Official Handbook of the Marvel Universe A-Z #2, the term "sliding timescale" is an attempt to quantify the passage of time in Earth-616 (the Prime Marvel Universe, where all mainstream stories take place) as a means of compressing time so that characters do not age noticeably.
Earth-616 featuring a sliding timescale means that rather than being fixed to any date in history, the modern era (which starts with the events of Fantastic Four #1 and continues on to the present) is condensed to a period of roughly 13 years that continuously slides forward in time. This means that at any time in the present, the spaceflight that gave birth to the Fantastic Four always happened around 13 years in the past. In 2007, the spaceflight happened around 1994. In 2017, it happened around 2004. In 2027, it will have happened around 2014. And based on that, all events in between are adjusted accordingly. The ratio of compression of time makes it so roughly 4 to 5 real-time years correspond to one year in Earth-616.
During Marvel publications in the 1960s and 1970s the passage of time on Earth-616 was described in the narrative as passing in semi-real time. Characters frequently made reference to what year it was, and often identified previous stories as having taken place in the span of months between publications. Characters were identified as having been involved in era specific military conflicts. For example, Mister Fantastic and The Thing were depicted as fighting in World War II, while Professor X of the X-Men was depicted as fighting in the Korean War. Many origin stories and events were also depicted based then current events that are considered dated or historical by today's standards. For example, the Fantastic Four attempted to fly into space to beat the Soviet Union in the space race, and Tony Stark was depicted as having become Iron Man while testing weapons in Vietnam during the Vietnam War. Many early stories and villains were inspired by the Cold War, and the eras' understanding of science and technology at the time.
By the late 1970s, Marvel had to start altering certain facts to prevent their characters from aging noticeably. One example of this was stating that The Thing was now a test pilot instead of fighting during World War II. With the number of real life spaceflights, the origin of the Fantastic Four was updated to explain how they were mutated while later astronauts were able to travel to space without ill effects, this explanation included solar flares and radiation passing through the Van Allen Belts, early references to the Space Race as a motivating factor were also excised.
In the 1980s, the passage of time was being marked in a slower progression. It was during this decade that the first "compressed" passage of time between the birth of the Fantastic Four and a story published at that time was actually spoken. In the 1990s, rehashing events that age the characters was often ignored. In the 2000s, certain concepts were either generalized, often when described in the most recent runs of the Official Handbook of the Marvel Universe and later depicted. Eventually a general standard for measuring time from Fantastic Four #1 to present publications needed to be established.
Marvel has since classified that all stories that are occurring in the "present time" that have been published after Fantastic Four #1 are part of the modern age (also known as the Age of Marvels, Age of Heroes, etc.) instead of identifying any specific decade or century, and this era is contained in a time frame of over a decade, with its beginning being set at a point always relative to the present.
The sliding timescale is not an exact science and is a matter of interpretation, and there are parts of the scale that just don't work when applied to the most minuscule detail. Trying to quantify the passage of time between all publications and all years makes quantifying a definitive timeline a matter of interpretation. Calendar specific seasons and holidays depicted in stories make it difficult to quantify this passage.
Likewise, key life events of specific characters also can often come to odds with any sort of measurement. Items such as a characters actual age, when they celebrate their birthday or when they have reached age specific milestones can cause irregularities. The sliding timescale is also affected by the fact that it didn't exist since the beginning of the time frame it covers. For instance, Peter Parker's sophomore through senior high school years ocurred over 2-3 years in real time, and this is considered as one of the sliding timescale's distinct aberrations.
The purpose of the timescale is to apply a general passage of time, focus on minuscule details are usually not addressed and regularly overlooked.
Topical References vs. Factual Reference
Certain facts, events, people of historical significance, pop-culture references, listed dates (such as the date on a newspaper headline), and sometimes even physical landmarks appear in comic books published years ago must be considered topical references relative to the date of publication so as not to prematurely age the characters or come to odds of the sliding timescale. As such the a reader should follow certain guidelines if they should accept these items as topical reference or a factual one.
A factual reference is one that cannot be refuted by the passage of the Sliding Timescale. They are events that are rooted to a particular era and the facts pertaining to these events cannot be subject to the timescale depending on when the story was published and what era of Marvel time the story is set in. For example, all Timely Comics stories that take place during World War II are all accepted as happening during the 1940s. Events depicted in this era are not subject to the Sliding Timescale, except for when a modern age story is measuring the passage of time between those events and the modern age.
A topical reference is a fact that is presented that gives the story context to the story as a frame of reference for the reader. These references are a product of the time the story is published and will become outdated with time. As such, modern readers observing such a reference from a story printed in a past decade -- for example someone in 2016 reading a comic book published in 1965 -- should never take these references literally. When describing these in a broader context -- such as describing the plot to a story or a characters history -- any references to these items should be at the very least generalized if not ignored.
The most common example is which individual is depicted as the President of the United States. These elements should be considered topical references. Topical References are facts that were true relative to the date that a given comic was published, and should be generalized when mentioned later. Since the publication of Fantastic Four #1 there have been about 13 presidential elections for the 50+ years of modern age publications. Based on the Sliding Timescale there should have only been 3 or 4, barring assassination, impeachment or other facts that might cut a presidency shorter than the 4 year term. However, various past presidents have been depicted as being the President of the United States during the modern age.
However, there are stories that have been published that have depicted a President operating in their appointed place in history as well as stories that were published during their tenure as president. For example, Richard Nixon has been depicted as the President in many modern age stories published between 1966 to 1976 starting with Incredible Hulk #119. These should all be considered topical references, especially considering the fact that Richard Nixon died in 1994. Whereas mentions of Richard Nixon in Marvel: The Lost Generation #7 should be considered factual references as they occurred in the 1970s of the Marvel Universe. Likewise, Nixon's appearances as a zombie in the modern age in Deadpool (Vol. 3) #3 should be considered factual as they are well after his death.
On this note, this means that appearances of Barack Obama in relatively recent publications (such as Amazing Spider-Man #583) will soon be considered topical references over time. Readers should get used to referring to these individuals as simply the "President of the United States" in a general sense instead of citing a specific individual.
Another example of topical references coming into play involves celebrities. For example, Strange Tales #130 features a story where the Human Torch and The Thing met the rock group known as the Beatles. While this was possible when the story was first published in 1965, this would be considered a topical reference now.
Another example are historical events being depicted in comics, the same event has been depicted as happening both in a specific year in the past, and in the modern age. The best example is the Apollo 11 moon landing. Fantastic Four #98, published in 1969, depicts the Fantastic Four stopping the Kree from disrupting this mission. This story is considered as happening in the modern age. However recent editions of the Official Handbook of the Marvel Universe have generalized this story. Instead of calling it the Apollo 11 space mission, they refer to it as a "manned spaceflight to the Moon." Conversely, Marvel: The Lost Generation #6, published in 2000, depicts the First Line preventing the Skrulls from interfering in the Apollo 11 mission. This story framed as taking place in the year 1969. Because the Fantastic Four story takes place in the modern age, the Apollo 11 landing in that story should be considered a topical reference and is generalized whenever it is mention in future publications. This story should be considered a factual reference because the story actually takes place in the year 1969.
Lastly, a more contemporary example of the Silding Timescale in motion is the depiction of the World Trade Center. Since Fantastic Four #1 the World Trade Center has been depicted as under construction as the original Twin Towers were completed in 1973. Since that date it has been predominantly featured as part of the New York City skyline in publications from 1973 to 2001. In 2001 the Twin Towers were destroyed in a terrorist attack. The skyline has not featured a building in that spot in publications until the complete of the new tower, One World Trade Center, in July 2013. What building stands in that location should be considered a topical reference in regards to any publication that depicts anything other than One World Trade Center in that location. That's because as of 2015, the Sliding Timescale has progressed that the modern age does not start until after 2001. The logistics of how this works is explained in the section below.
Often times there are flashbacks that apply the publication date to events that have happened in the past that are subject to the Sliding Timescale. For example All-New X-Men Annual #1 shows a scene where a trip through time ascribes the Fantastic Four's first battle with Galactus to the year 1966. This was the year of publication. This should be considered a homage to the original story's date of publication and not taken literally.
Another example is the dates on Adam Warlock's tombstone in Marvel Two-In-One Annual #2 and other publications that specify a specific date. In that story the dates of Warlock's life are documented as 1967 to 1977. These dates coincide with the publication dates between his first appearance and his (then current) death. These dates should be considered topical. When applying the Sliding Timescale Warlock would have been alive and active for two years.
In Ultimates (Vol. 3) #5, Galactus states that the time-space continuum is much more malleable than humans believe. The events that change history have a peculiar weight and are dragged in the wake of the present, positioning events that happened a long time ago merely a handful of years into the past.