Is Sicut Dudum reliable?

Pope Eugene IV supposedly promulgated the papal bull Sicut Dudum in 1435.  We have the text of the bull in Latin with the date January 16, 1434 (which would be in 1435 by our reckoning) or December 17, 1434.  However, I am in doubt as to the authenticity of this, both because of the discrepancy in the date between the Latin and Italian, and because we don't know where the original document is.  Maybe somebody knows, but it isn't on the Wikipedia page in English or Italian.  Is it in the Vatican Library? Then why not say so?  Why do people just assume that Wikipedia (or Encyclopedia Britannica) is correct?  So we have the text in Latin, but where did it come from?

Along the same lines, did you know that Pope Eugene IV had only 340 books, and the Vatican Library was really founded by his successor Pope Nicholas V (or perhaps Pope Sixtus IV)?

Even if you assume that Sicut Dudum is a reliable document (and I have doubts about that), it seems we really have come to the end of history.  Of course lots of events happened before that, but these other things lie in the realm of legend and story-telling.  Which doesn't mean that they didn't happen, it just means that we don't know exactly when they happened, or how much of the story is true.  And of course we have exact dates of when events supposedly took place, but what is our source?

So for now, I think anything prior to 1455 is in doubt.   The fall of Constantinople was in 1453.  So I guess that is the date to choose.  We don't know ANYTHING with certainty prior to this point.

But what about Eastern Christianity and Russia, don't they have dates before that?  Yes, but they used a different calendar system called Anno Mundi, which dated from the creation of the world according to the Septuagint. "The Byzantine [Anno Mundi] era was used as the civil calendar by the Byzantine Empire from AD 988 to 1453".

At some point our current western calendar was arbitrarily imposed.  As in, they just started numbering years from 1453 or whenever.  And then the amateur historians of the day filled in the history with their imagination based on legends they had heard.  And then the received history made its way in to school textbooks and wikipedia and here we are.