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) continuously slides forward in time.
This concept dates at least to the 1980s, when John Byrne revealed in David Anthony Kraft's Comics Interview #25 that Marvel had set a period of 7 years between Fantastic Four #1 and the new stories being released at the time.
As per 2008's Official Handbook of the Marvel Universe A to Z #2, this period is roughly 13 years. This means that, at any time in the present, the spaceflight that gave birth to the Fantastic Four always happened around thirteen years ago. In 2008, that would be 1995. By 2012, the space flight happened around 1999. And based on that, all events in between are adjusted accordingly. The current ratio of compression 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 based on 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 in real-time. One example of this was stating that The Thing was now a test pilot instead of fighting during World War II. With a large 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 Marvel first created, as John Byrne described, a "sliding scale" that would move forward in time.
In the late 2010s, the problem of character origins tied to real-world wars was seemingly fixed for good after the release of History of the Marvel Universe (Vol. 2) #2, which introduced the Siancong War, a fictional conflict that would move forward in time alongside the Marvel heroes.
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 character's 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 occurred over 2-3 years in real-time, and this is considered as one of the sliding timescale's distinct aberrations.
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 that appeared 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 reader should follow certain guidelines if they should accept these items as a topical reference or a factual one.
A factual reference is one that cannot be refuted by the passage of the Sliding Timescale. There are events that are rooted in 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 product of the era 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 the present 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 character's history -- any references to these items should be at the very least generalized, if not ignored.
The most common example is the President of the United States. Since the publication of Fantastic Four #1, there have been more than a dozen presidential elections in the real world, but only a fraction of the time has passed in the Marvel Universe. Thus, 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.
For example, Richard Nixon was 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. 5) #3 should be considered factual as they're set well after his death.
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.
A different example is historical events being depicted in comics. For instance, 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, which means it should be interpreted as a topical reference. 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 is framed as taking place in the year 1969 and should be considered a factual reference because the story actually takes place in the year 1969.
Lastly, a more contemporary case of the Sliding Timescale in motion is the depiction of the World Trade Center. After the completion of the Twin Towers of the World Trade Center, in 1973, it was predominantly featured as part of the New York City skyline. In 2001, the Twin Towers were destroyed in a terrorist attack. Therefore, the comics released after the completion of the new tower, One World Trade Center, in July 2013, feature that building instead. 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, because the Sliding Timescale has now progressed so that the modern age does not start until after 2001.
There are often flashbacks that apply the publication date to events 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.
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.