The UI stores wrong date values for Julian calendar model dates. It doesn't convert them to proleptic Gregorian date values. For the same reason, it doesn't display correct values. It doesn't default to Julian for dates before 1500.
All this was working well some time ago.
Example:
Date of death of Alexander the Great https://www.wikidata.org/wiki/Q8409 10 June 323 BC (Julian) was entered when the UI worked well and stored correctly 5 June 322 BC (proleptic Gregorian). Now it displays wrong 5 June 323 BC. On sandbox you cannot enter 10 June 323 BC (Julian) and store 5 June 322 BC (proleptic Gregorian).
The old function should be restored.

Version: unspecified
Severity: major
Whiteboard: u=dev c=frontend p=0
See Also:
https://bugzilla.wikimedia.org/show_bug.cgi?id=70395
https://bugzilla.wikimedia.org/show_bug.cgi?id=65847