I read today that on this day in 79CE, the famous Pompei eruption took place, and it got me wondering. I know that Romans used the Julian calendar which is similar to our Gregorian calendar save for a skipped day every 100 years or so.
I also know that in Britain, the switch from one calendar to the other caused some date duplication, as in I know Isaac Newton was born on december 25th/January 4th, depending on which calendar you look at.
So, my question is this: When historians say "The eruption that buried Pompei took place on August 24th in the year 79CE", do they mean using the Gregorian calendar or the Julian calendar? Does this same answer apply for the entire era between the introduction of the Julian calendar and the switch to the Gregorian?
It depends on the individual historian to an extent -- I see this come up in my field when I'm reading about the Anglo-Dutch wars in the 17th century, when much of Europe had switched calendars but the English had not. In general, you see historians of this era giving dates as either Old Style or New Style; for example, the naval battle called the Four Days Battle was fought 1 June to 4 June 1666 in the Old Style calendar, and 11-14 June in the New Style calendar. Usually you just see them identify it at the start and continue that way.
This also comes up in histories of e.g. the Pacific theater in World War II -- in Japanese records the Pearl Harbor attack occurred on Dec. 8, 1941, as the fleet kept Tokyo time, even though at Pearl Harbor it was the early morning of Dec. 7 when the attack happened.