It turns out that Darwin's date formatting uses the Julian calendar
for dates before 1582-10-15, when the Gregorian calendar first came
into use (in some countries, while many others continued using
Julian). This leads to discrepancies between the (Gregorian) dates we
pass it for formatting and the (Julian) dates it actually prints, that
are the same number of seconds before 1970.
Previously the QLocale system backend for Darwin already had a kludge
to work round its handling of negative years, so it suffices to extend
that to years before 1583.
Fixes: QTBUG-54955
Pick-to: 6.6 6.5
Change-Id: I70f219b73bf20c0cd63bcda2b0e99042354872ca
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
207954d5f0