Upvote:10
A modern secondary historical paper or book should always use the Gregorian Calendar and the era of Exiguus. For pre-Gregory dates the proleptic calendar should be used. The reason for this is so that the exact distance between two dates can always be calculated easily and so that all books are using the same time scale so that dates in one book can easily be compared to those in other books.
In cases where a date from a source document is being used, then that date should be given verbatim, as it appears in the source document with no interpretation or conversion, along with the era and calendar being used so far as it is known, followed by the believed Gregorian equivalent. For example,
...according the MSS, the battle occurred a fortnight from Michaelmas, 1143 (AD, Julian), which is 14 October (Gregorian),...
The reason why you find Julian dates in many history books is because the historian is simply copying the date from the source without further comment, and in many cases without identifying the source, which often may be some other secondary source whose author is, in turn, doing the same thing. Because of this, most history books are littered with chronological mistakes of various kinds.
If you are drawing a date from such a book and using it, you should identify what calendar you believe the author to be using.
...the naval engagement occurred on 15 March 1743 (Julian?)[1]...
So, in this case did the author of the book you are reading translate the date to "new style" or is it an "old style" (Julian) date? You make your best guess (Julian?) and footnote the source. Sometimes you will see ("o.s.") in books, indicating that they are using an old style date.