(This was originally posted in r/history before I realized that this may be a better place to post this.
Which calendar is used for older dates (i.e. before the adoption of the Gregorian calendar) as referred to in modern history? For example, if in a history textbook mentions the date 27 January 535 AD, is it using the Julian or Gregorian calendar? What about the traditional date of the founding of Rome, 21 April 753 BC? In that case, does the date use the Julian or Gregorian calendar or some older calendar?
Practicing historian here. (Moving a lower level comment up and expanding some)
I work in the period both before and after the Gregorian calendar was adopted in the Spanish Empire (1582). Yes you could adjust dates for events occurring under the Julian calendar but then you would be using different dates than those found in the primary sources. Even though I work on both sides of the calendar reform, I never adjust dates. There is no reason to. The dates in the documents are correct.
Remember the switch from the Julian to the Gregorian Calendar ultimately involved skipping a specific number of days to account for drift in seasons since the establishment of the Julian system. The Julian calendar did not precisely account for drift caused by our calendar year of 365 days not mapping on to the actual orbital time of the earth. This means that the switch between the two calendars necessitated a skipping a head of days specific to the time in which the switch occurred. In 1582, the Spanish Empire skipped ahead 10 days in October of 1582. The English didn't switch until the mid 18th century by which point the calendars had drifted to 11 days difference. This also means that calculating a Julian date within the Gregorian system requires variable adjustments. Earlier dates, dates closer to the establishment of the Julian calendar require fewer days difference than modern dates which require more.
They only time it would make sense to adjust dates in a historical work would be when doing comparative history where one area understudy retained the Julian calendar while the other adopted the Gregorian. In that instance clarity might demand express correlations. A classic example is the death of Shakespeare and Cervantes, great authors of the late 16th century. Both died on April 23, 1616. Yet, they actually died 10 days apart. When Cervantes died the date in the Spanish Empire was Gregorian, and so 10 days before the day in which Shakespeare died in the Julian.
So to reiterate, historians use the date as recorded in the documents. If necessary adjustments can be made to help clarify in situations when dates between two places come from different calendars. Historians do not as a rule change Julian dates to Gregorian dates. The Battle of Tours occurred on October 10, 732. We do not adjust that a few days forward to be Oct. 14.