When a modern historian is writing about European history in the English language, I know it is customary to use either the Julian or Gregorian calendar, whichever was in force in the time and place being written about.
But what about non-European countries that went directly from an indigenous calendar to the Gregorian calendar, and never used the Julian calendar? For example, Japan switched from a traditional luni-solar calendar to the Gregorian calendar in 1873. So if an English-speaking historian were writing about Japan, would she use the Gregorian calendar for events as recent as 15 October 1582, and the Julian calendar before that? Would the Gregorian calendar be used even before it was officially created when writing about events in Japan?
Any references to accepted sources on how historians should write would be appreciated.