I don't know what the professional practice at present is, but I do 
know that in the 18th c. in England (and America) a lot of dates were 
written double -- so that George Washington, for instance, had a 
birthdate of Feb 11/22 -- Feb 11 Old Style (Julian), Feb 22 
(Gregorian) New Style.

(In passing, I might note that England's anti-Catholicism is, as this 
correspondance shows, still wrecking havoc in the 21st century.  The 
English kept the Julian calendar because the Gregorian calendar was, 
well, Roman Catholic .....)

Peter G. Stillman