Slightly related, correlating different ancient calendars that weren't ever explicitly lined up at the time in historical records is a delightful, if esoteric topic. While there may not be any missing time from recent European history, it's a lot hazier if you go even further back.
For example, Mesopotamia laid dead for 3000 years before being rediscovered. Their civilizations were long gone when the Greeks and Romans started leaving written texts. Akkad and Sumeria were rediscovered anew in the early 20th century, even more completely anew than Ancient Egypt had been a century before that. Along with that came uncertainty of the dates and calendars. Even today, we are still not sure of the dating of Old Babylon, let alone the first cities of Sumeria another thousand years before that. Today there are three competing interpretations, attempts to to align the day-by-day written records (often not overlapping from varying locations) with a constant chronology. Was Hammurabi born in 1810 BC, 1760 BC, 1830 BC? We have exact dates given for when he did things. But we don't know what those refer to! Similar ambiguities exist for Ancient Egypt. [1]
That's not the case with the Mayan long calendar! They were diligent astronomers, more so than the Babylonians, if we go by what records they left of observed events. The body of evidence over the last ~30 years has mounted to become nearly irrefutable. There's only one possible concordance that fits, and it fits the date exactly. To the point it's safe to say it's historical fact that Pakal the Great was born 9.8.9.13.0 8 Ahaw 13 Pop (19 March 603 AD), acceded to the throne 9.9.2.4.8 5 Lamat 1 Mol (26 July 615) and died 9.12.11.5.18 6 Etzʼnab 11 Yax (26 August 683).
For example, Mesopotamia laid dead for 3000 years before being rediscovered. Their civilizations were long gone when the Greeks and Romans started leaving written texts. Akkad and Sumeria were rediscovered anew in the early 20th century, even more completely anew than Ancient Egypt had been a century before that. Along with that came uncertainty of the dates and calendars. Even today, we are still not sure of the dating of Old Babylon, let alone the first cities of Sumeria another thousand years before that. Today there are three competing interpretations, attempts to to align the day-by-day written records (often not overlapping from varying locations) with a constant chronology. Was Hammurabi born in 1810 BC, 1760 BC, 1830 BC? We have exact dates given for when he did things. But we don't know what those refer to! Similar ambiguities exist for Ancient Egypt. [1]
That's not the case with the Mayan long calendar! They were diligent astronomers, more so than the Babylonians, if we go by what records they left of observed events. The body of evidence over the last ~30 years has mounted to become nearly irrefutable. There's only one possible concordance that fits, and it fits the date exactly. To the point it's safe to say it's historical fact that Pakal the Great was born 9.8.9.13.0 8 Ahaw 13 Pop (19 March 603 AD), acceded to the throne 9.9.2.4.8 5 Lamat 1 Mol (26 July 615) and died 9.12.11.5.18 6 Etzʼnab 11 Yax (26 August 683).
[1] https://en.wikipedia.org/wiki/Chronology_of_the_ancient_Near...