I had never considered converting calendars. It is called Dual Dating. I looked at Wikipedia's events for August 3rd and it is the 215th day of the year (216 in leap years) in the Gregorian calendar. Then it lists events starting with the year 8 when Tiberius defeats the Dalmatae on the river Bathinus. So if the Gregorian calendar began in 1582, I wonder how they did all these backward calculations. I expect it was monks in Monasteries.
"The original goal of the Gregorian calendar was to change the date of Easter. In 1582, when Pope Gregory XIII introduced his Gregorian calendar, Europe adhered to the Julian calendar, first implemented by Julius Caesar in 46 B.C."
There's a site on the topic of dual dating between various Calendars. It's at: