It is a little surprising that historians would not know about this.
Your final question -- what is professional practice?
is a good one (and I don't know).

Not all countries changed their calendars at the same time --
not even in Europe -- In fact the American colonies changed in 1752
(I am depending on Wikipedia for that) so that even the birthdays
of George Washington, Thomas Jefferson, et al
are affected by the change.

Martin C. Tangora