"It must have been, then, that there were local attempts to retain the coincidences between the true and the calendar year — intercalation of days or even of months being introduced, now in one place, now in another ; and these attempts, of course, would make confusion worse confounded^ as the months might vary with tlie district, and not with the time of year.

That this is what really happened is, no doubt, tlie origin of the stringent oath required of the Pharaohs in after times, to which I shall subsequently refer.

[...]

When the year of 365 days was established, it was evidently imagined that finality had been reached ; and, mindful of the confusion which, as we have shown, must have resulted from the attempt to keep up a year of 360 days by intercalations, each Egyptian king, on his accession to the throne, bound himself by oath before the priest of Isis, in the temple of Ptah at Memphis, not to intercalate either days or months, but to retain the year of 365 days as established by the Antiqui.^ The text of the Latin translation preserved by Nigidius Figulus cannot be accurately restored; only thus much can be seen with certainty.

To retain this year of 365 days, then, became the first law for the king, and, indeed, the Pharaohs thenceforth throughout the Avhole course of Egyptian history adhered to it, in spite of their being subsequently convinced, as we shall see, of its inadequacy." https://archive.org/stream/dawnastronomyas00lockgoog/dawnast...

Edit: I have no idea how accurate any of this stuff is. From my little experience trying to figure out the origins of historical claims I have learned it is very difficult to do so. Poor citing practices abound.

Failing to record eclipses doesn't seem very remarkable. There are on average about 1.5 lunar eclipses per year. Clearly people throughout history were not recording every single one. Even the densest parts of your graph are missing most of them.

Edit: also note that this graph is "timed" eclipses only. I take that to mean that it only includes eclipses which were not only observed and recorded, but for which the record includes the local time of day of the event. This is necessary for measuring the Earth's long term change in rotation, as that slide does, but unnecessary for synchronizing calendars.

On the off chance you are familiar with this type of research, do you have any conception for how accurate dates like these are and how they were arrived at? I find it very difficult to find out what the ground truth evidence is for any historical (ie pre-printing press) data.

Pretty much. I start to doubt the importance of what I read when it becomes difficult to figure out where/what/when of the underlying data. At that point it seems ok to wildly speculate and see if any testable predictions come out of it.

I haven't put forth that much effort at this (it's more of an interesting "waiting for something else to finish" type task), but still find it curious I have had so much difficulty following, or even finding, citations to the original sources.

Imagine that applied on a large scale after a revolt or war. The enemy would be so vilified a law would be passed to wipe the time they were in power from history.

Then later generations try to piece together what happened and can't tell whether that "forgotten" dynasty lasted 100 or 200 years, but they do their best and guess 200 years (nope it was 100). Then we base our history off records of that earlier guesswork.