First officially referenced in 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) and other similar universes as a means of compressing time so that characters do not age noticeably. While not considered when Marvel Comics first began publishing, it became apparent that something needed to be done to logically explain the flow time and how characters remained in the same age span as they were first introduced despite living through years of published stories. This practice began in the late 1970s and early 1980s. A full definition and explanation was made in 2007 in the aforementioned Official Handbook of the Marvel Universe A-Z #2. The Sliding Timescale is used to keep track of the time that has passed during a period that is now coined as the Modern Age which covers events from Fantastic Four #1 to current publications.
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 as their characters were being prematurely aged. 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 space flights, 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 publications being published 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, the Modern Age of Heroes, etc.) instead of identifying any specific decade or century. As of 2016, roughly 14 years have passed since the birth of the Fantastic Four. This will be considered more or less the generally accepted passage of time until about late 2017.
In 2008, Marvel had officially stated  that for every four to five years of real time publications, one year of Marvel time had passed on Earth-616 and other time-sensitive realities. Based on a 4:1 radio model, the Modern Age of Heroes has been on going for about fourteen years based on the following model:
Year 01: Nov 1961-Oct 1965
To clarify: While a year of Marvel time is a rough count of 365 days to a year, this passage of "one year" does not follow the Gregorian calendar from January 1 to December 31, but a full year from the date of the space flight that created the Fantastic Four. The specific calendar date the Fantastic Four went on their fated space flight has remained vague. Although one source states it happened in the summer.
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 best example of these difficulties can be gleaned when trying to determine how many years it took Peter Parker to graduate from high school. For example, Amazing Spider-Man #533 states that Peter was 15 years old when he first gained his powers in Amazing Fantasy #15. Peter is shown graduating from high school in Amazing Spider-Man #28. Amazing Fantasy #15 was published in 1962, while Amazing Spider-Man #28 was published in 1965. Based on the 4:1 ratio of the Timescale Peter had been operating as Spider-Man for roughly a year. According to Wikipedia the average student in New York City must complete 12 grades of education. Grade 12 being the final year of high school, the average age at graduation is 17-18 years old for that grade level. Based on this logic, if Peter was an average student he would have had to have completed at least another 2-3 years of schooling before reaching his diploma based on these real life guidelines. However, one could assume that due to Peter's level of intelligence and focus on his schooling he may have advanced in school sooner than others the same physical age.
The above example clearly illustrates that any measurement of the Sliding Timescale should be taken with a grain of salt. However, while there may not be an official explanation to quantify these irregularities, they can be interpreted at the whim of the reader.
However the purpose of the timescale is to apply a general passage of time, focus on such minuscule details are usually not addressed and regularly overlooked.
Another factor of consideration is the process of time compression. It is a common misconception that time in the Marvel Universe is constantly compressing, meaning that the events of following Fantastic Four #1 continue to compress so that less time has passed from that story to the present. This is not correct.
What is meant by "time compression" is that compression of four years of publications into a single year of Marvel Time. During a four year span of publications characters will reference events as though they happened in "real time." They may refer to a specific event happening a few months ago, or a year prior. However, after a certain period stating the physical number of years puts too much time between the present and past events and adds more time to history, cancelling out the purpose of the Sliding Timescale. As such, after four years of publications the events are condensed into a single year of "Marvel Time." Further references of those events would then adhere to the Sliding Timescale measurement as opposed to real time.
Civil War and Secret Invasion, per the sliding timescale occurred in "Year 12," of the sliding timescale. At the time Secret Invasion was being published the characters referred to the events of Civil War as happening over year ago. That measurement should be considered topical since it actually gives perspective of time between the two events via publication giving casual readers a point of reference between the two events.
However as soon as November 2009 elapsed and a new year of "Marvel Time" began, publications from November 2005 to October 2009 are compressed in a single year of "Marvel Time." These four years of publication signify one year. As such publications in the 2016 year the events published in Nov 2005-Oct 2009 should seen as happening about two years ago. This will remain the case until the next calendar roll over of November 2017, then those events get bumped back a single year.
To put this into perspective the timescale compresses time as so:
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.
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. This could also be another instance where reader interpretation can also be used to explain a situation. As of 2016, Marvel hasn't referenced this story. Whereas typically they would generalize the Beatles -- identifying them as a "popular British rock group" for example --- one could argue that these are actually the Skrulls who have posed as the Beatles. However that is a matter of interpretation and has no factual grounds at this time.
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.
Computing Marvel Time
Remembering that the measurement of Marvel time is a matter of interpretation, one should always refer to said passages of time as a rough estimate and that various calculations could differentiate between years depending on how the calculation is depicted. Some readers will apply a strict four or five year passage of time to events, while others will consider other methods of time passing. The most prevalent being that the first year or two of Marvel publications between November 1961 and October 1963 actually occurred in "real time" and all other publications adhere to either a 4:1 or 5:1 ratio. For the sake of providing a clear example, this computation is using a strict 4:1 ratio timescale.
In order to calculate time, one must consider the current publications as happening this year. For example, comic books released on October 21, 2015 -- on the day they are published -- are considered as happening in the year 2015. The year 2015 falls in "year fourteen" of the modern age, which is slated to last until roughly October 2017. As each year elapses the story in October 2015 is taking place in 2016 when its the year 2016 and could be considered as happening a number of weeks or a few months prior, and happening in 2017 when that year elapses and be considered as happening a few month to half a year prior. The "year" that story occurred in would slide forward until October 2017. Once most of 2017 elapses and "year fifteen" begins in November 2017, then the story published in October 2015 begins to slide backward in time, making it an event that happened about a year prior. As it is characterized as "year fourteen" it slides backwards as Marvel time moves forward.
Applying this principal to past years can help give a rough idea of how many years has elapsed between periods of time.
Example #1: How many years passed since Captain America was revived in the modern age?
In Avengers #4 it was identified that Captain America was placed in suspended animation in April 1945 and revived in the modern age. That story was published in 1964 and states that Cap was in suspended animation for around 20 years. That reference of time is considered a topical reference based on the year that story was published. As the Sliding Timescale moves the amount of time between the fixed year of 1945 and the year Captain America is revived in will constantly be increasing.
If the present of Modern Age is 2016, and it is considered year fourteen of the Marvel Sliding Timescale, the time passing would be computed as so:
Avengers #4 was published in 1964. This falls under "year one" of the 4:1 timescale ratio. Today is 2016, the year 2016 falls in "year fourteen" of the 4:1 timescale ratio.
From 2016's perspective, Avengers #4 took place roughly fourteen years ago. So roughly the year 2002.
Cap going into suspended animation happened in 1945. So 2002 - 1945 = 57 years had passed between 1945 and Avengers #4. Based on 2016's publications; however, Captain America was put in suspended animation about 71 years ago.
When "year fifteen" draws to an end in October 2021, the time scale will have moved forward. The "present" will then considered 2021. Cap's freedom from suspended animation slides forward roughly one year. 2021 - 15 years = the events of Avengers #4 now happen in the year 2006; however, his time in suspended animation would now have lasted 61 years, and the time between 1945 and the "present" would then be 76 years.
Example #2: Determining a Topical Reference:
With the passage of time certain events, people, and places become topical references. One of the most prolific of these events was the terrorist attack on the World Trade Center on September 11, 2001, which saw the destruction of the original twin towers (originally constructed in 1973) collapse. a new tower was constructed in its place in 2013.
It's a unique example of how the topical references are determined in the Marvel Universe. Because it was not constructed until 1973, it's obviously absent from all "Modern Age" stories from 1961 until it began construction. It was first depicted being constructed in Daredevil #82 published December 1971 and first depicted fully constructed in Astonishing Tales #18 published June 1973. Since that story it has regularly appeared as a staple of the New York City skyline in many Marvel publications. The terrorist attack was later depicted as happening in the Marvel Universe in Amazing Spider-Man (Vol. 2) #36 published in December 2001. The new tower constructed at One World Trade Center was completed in July 2013.
Because of the Sliding Timescale, all events published in "year one" of the Modern Age now happened about fourteen years in the past. The farthest back the Sliding timescale goes now is 2002. As such all references and depictions of the Twin Towers in the present tense should be considered topical references. Moving forward depictions of New York between the destruction of the original Twin Towers and the completion of One World Trade Center will be considered topical references during "Year Eighteen" of Marvel time around the year 2030. However readers should get into the habit of considering references to the terrorist attacks and subsequent reconstruction of the World Trade Center site as a topical reference now.
Example #3: Determining the passage of time pre-Modern Age
The Sliding timescale pushes forward the past lives of characters prior to the modern age forward in time as well. The best example of this is references to Mister Fantastic fighting in World War II as seen in Fantastic Four #11, yet Amazing Spider-Man #535 depict Reed as a child during the McCarthy Era of the 1950s. Both depictions should be considered topical references. One should ignore the era-specific references to any time a characters past is visited prior to the Modern Age.
However the passage of time from an instance in the "pre-modern age" to the modern age can be computed by determining the characters relative age in that pre-modern point in time to a point in the modern age.
Using this logic, we can determine the passage of time between Peter becoming an orphan and becoming Spider-Man can be easily determined:
When Peter gained his powers in Amazing Fantasy #15 he was fifteen years old. Peter was still an newborn when his parents were killed, as depicted in Amazing Spider-Man Annual #5. One can assume roughly 15 years had passed.
If "today" is 2016, Amazing Fantasy #15 (published in 1962) happened during "year one" of the modern age, or about fourteen years ago.
So by 2016's date, Peter gained his powers in the year 2002, and he was born sometime around 1987 or 1988.
When "year fifteen" draws to an end in October 2021, the time scale will have moved forward. The "present" will then considered 2021. Peter will have gotten his powers in 2006 and he would have been born around 1991 or 1992.
From the perspective of "today", if today is the year 2016, Peter was orphaned about 29 years ago, but when "today" is 2021, then he will have been orphaned around 30 years prior.
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. 2) #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.