ChatGPT gets Maya dates wrong. I have asked it to convert Maya dates into Gregorian, and it always gets them wrong. When I tell it what the correct date is, it apologizes and tells me I am right.
John Linden and Victoria Bricker, the authors of the article being discussed, are well-respected scholars. So these are not the normal nut-jobs that talk about the Maya Calendar. I have yet to read more than the summary, but will read the article in the next few days. I am sure that Linden and Bricker got the math and astronomy right. However, I am very suspicious that this is how the Maya actually used the 819 day count. There are only about 21 of these 819 day counts recorded in the Maya inscriptions, so proving anything about these is very difficult. 819 is the multiplication of 7x9x13, three periods frequently used by the Maya, so it is more probable that it is just a larger period that comes from these three smaller periods.