The sequence of past events in the history of Dawnguard is not easy to discern. There are a few vague hints, but also contradictions. No specific years are mentioned, but at least a few chronological fragments are provided. Together with the historical facts of the known lore, one could attempt to solve the puzzle and, in doing so, identify nonsensical statements.

After awakening Serana, probably every player has asked themselves how long she had been imprisoned. At first, she herself cannot tell how much time has passed, and later she can only hint at it.
The first conversation with her is important because everything she says has not yet been influenced by impressions of the present. First, it is noticeable that she has no knowledge of an Empire. However, she does know the name "Cyrodiil" (at least she repeats the name without asking, as if the term were familiar to her). When did a Cyrodiilic Empire emerge? Reman I unified the provinces 1,700 years ago. Therefore, Serana must have been imprisoned before that.

She comes from a noble family and appears to be well-educated. Therefore, she should know from history lessons that there had already been an earlier Empire originating from the Heartlands. Under Empress Alessia, the Elves were defeated or driven from the Heartlands, resulting in large parts of Tamriel being unified into an Empire. However, at that time, the land still bore the ancient name "Cyrod". The name "Cyrodiil" only appeared shortly after Alessia's death and is closely tied to the Alessian Empire.

Thus, Serana's statement is already strange here — she is familiar with Cyrodiil, but not with an Empire.

To straighten this out, one would have to assume that "Cyrodiil" was already in use as the name of the Heartlands before Alessia. There is nothing to contradict this; it is a plausible assumption. And with that, Serana's statement also becomes plausible. According to this, she was imprisoned at least 4,200 years ago.

Serana makes a perplexing statement at the beginning of the quest "Prophet". When considering where to find a Moth Priest, she suggests checking at the College of Winterhold. She knows that it is a center of learning and that mages run the place. Did the College really exist 4,200 years ago? That would be a new revelation. The question then would be who was in charge of the College back then. The Mages Guild was only founded 3,000 years later. Therefore, it could only have been the Psijics or a still unknown association of mages.
However, it seems more plausible that Serana only recently learned of the College's significance and existence. In the case of choosing the Dawnguard side, she makes this statement inside Fort Dawnguard — meaning she has traveled across the entire country and could have picked up this information along the way. If one assumes this, the timeline remains at 4,200 years.

What happened after her imprisonment? Her mother, Valerica, traveled to the Soul Cairn and was imprisoned there. After that, the dragon Durnehviir appeared there and was forced by the Ideal Masters to serve as a guardian. Durnehviir states that he sought out the Soul Cairn to attain even greater power in order to assert his territorial claims against his fellow dragons. This means that at that time, dragons ruled the land and the skies. It also implies that the alleged Dragon War (alleged because it was suddenly introduced with TES V) had not yet taken place. Therefore, it was in the late Merethic Era.

Consequently, the time of Serana's imprisonment must be adjusted to at least 5,000 years ago.

However, the timeline is now becoming tight, as Ysgramor landed in Skyrim with the first Nords only a few centuries earlier. This would still be plausible, as Dimhollow Crypt could have been built by the early Nords. Nevertheless, it seems doubtful that the term "Cyrodiil" already existed back then. The Ayleids ruled the Heartlands.

5,000 years and "Cyrodiil" do not match. Evidently, we have an anachronism by Bethesda here.

Furthermore, it is a mystery why Serana does not recognize her surroundings after awakening. The stone monolith in which she was enclosed architecturally matches the entire ceremonial structure. Consequently, the structure must have been created at the same time as the stone monolith, especially since they form a single unit. There is nothing else in the cavern room that could have been added later and which she would not know. A possible explanation would be that Serana was brought to Dimhollow Crypt while unconscious, and therefore sees everything for the first time upon awakening. Strange, but it does not help any further in determining the timeline.

Now let us consider the two Snow Elf brothers. Vyrthur states that he became a vampire long before Serana's birth. This is also possible, as the Snow Elves (an Aldmer offshoot, similar to the Dwemer) were in Skyrim before the Nords. However, his brother Gelebor states that the Chantry of Auri-El was "built at the beginning of the First Era". If Vyrthur served there as Arch-Curate from the very beginning and became a vampire shortly thereafter, then one would have to adjust Serana's imprisonment back to 4,300 years ago. But that cannot be, because by that time, the Dragon War would already be over, and Durnehviir's story would no longer be chronologically coherent.

Another derivation: Vyrthur becomes a vampire while serving as Arch-Curate of the Chantry. The Chantry has existed since the beginning of the First Era. Therefore, he creates his prophecy 4,200 years ago at the earliest. After that, the Elder Scroll circulates and ends up in Castle Volkihar, where it is read and Harkon succumbs to the prophecy. Valerica takes the Elder Scroll, locks Serana away, and travels to the Soul Cairn. But that cannot be, because according to Durnehviir, Valerica was already there before the First Era and before the Dragon War.

Consequently, Serana would have been locked away before Vyrthur's birth. Vyrthur, on the other hand, claims that he became a vampire before her birth...

4,300 years (based on the statements of Vyrthur and Gelebor) does not match the statements of Valerica and Durnehviir. These statements ought to coincide. Instead, they differ by many centuries. Was Bethesda asleep at the wheel?

Viewed from another angle: Gelebor states that he has been serving in the Chantry for thousands of years. Unless he was specifically blessed with immortality by Auri-El (which is nowhere implied), this statement is questionable. He is not a vampire like his brother. His life expectancy is finite. Although Elves live much longer than humans, they do not live for over 4,000 years, especially since Gelebor is not an old man and apparently in the prime of his life. I would estimate his age to be around 600 years.

However, Gelebor also states that the magic of the Wayshrines was lost long before his arrival here. Therefore, he was not in the Chantry from the very beginning, which opens the door for the following consideration:
If his brother Vyrthur is the same age as he is (i.e., around 600 years old) and one assumes that he became Arch-Curate in the Chantry at the age of 100 and a vampire shortly thereafter (which, according to his statement, was before Serana's birth), then Serana's birth could only have occurred in the mid-Third Era. Not much earlier. This time estimate is also corroborated by Vyrthur's statement that he spent centuries (not millennia) preparing for the arrival of the Daughter of Coldharbour.

But that would turn the previous deductive results upside down. Here again, Bethesda was not paying attention.
Because one fact cannot be bypassed: Valerica has been trapped in the Soul Cairn since the Merethic Era.
She explains this plausibly, and Durnehviir confirms it. This is also substantiated by Serana's later statement that "a few more thousand years of obsession haven't exactly done [her father] any favors".

Therefore, Valerica only had the opportunity to lock her daughter away in Dimhollow Crypt prior to that. Accordingly, it was over 5,000 years ago.

Since both women have Elder Scrolls containing the prophecy with them, Vyrthur must have written the prophecy even earlier. He demonstrably did so in the Chantry of Auri-El. Only... the Chantry did not exist back then. And neither did Vyrthur. Whichever way you turn it or from whichever side you look at it — the chronological sequence contains contradictions, and Bethesda confuses the player with incorrect statements. Apparently, several designers worked on this and did not coordinate with one another. Unfortunately, part of the magic is lost as a result.