This Day in History is about Berta

Sometimes I look at this day in history, and today I wonder how it is decided that it is on this day in history, given all the calendar date shifts that we have experienced.  there must be experts for this task. This day in history - July 12 783        Jul 12, Bertha "with the great feet", wife of French king Pippin III, died. Priscilla Howe tells us a little  about Bertha (Berta) in her blog - storytelling notes.blogspot.com HERE  Her synopsis is: "Queen Berta's identity is stolen, she's taken to the forest to be killed, she escapes only to face brigands, bears and brambles. How will her big feet save her from this terrible treachery?" On the other hand, Wikipedia propositions that she may have had a clubfoot, as they indicate her nickname is Bertha Broadfoot rather than 'with great feet'. Today I am wondering how we know she died on July 12th, 783.   AskHistorians has this question:  It must be really confusing with all these different systems. When one source for a date concerning an event is in the Jewish calendar, another in the Arabic calendar and another in the Gregorian. The answer:  Usually historians can convert dates from entirely different calendrical systems into their Gregorian equivalents as needed; for example historians writing on Islamic history will often list dates in both the Islamic calendar and their Gregorian conversion and Mesoamericanists can use an established correlation(most notably the GMT correlation used for Mayan long count dates) to convert Long Count or Calendar Round dates into their Gregorian equivalents.  This sounds like a lot of mathematical equations and calculations.  To me it looks like too many to hope they got them all correct.  To the person who answered the question,  it seems routine.   I can tell you that everyone loves to see bees on Lavender.  I've been told by Lavender growers that bees are known to fall asleep amongst the Lavender.  It is considered to have calming and sedative effects for humans as well.