It's a real pain if you want to translate exact dates from a different calendar into the common era one. Specifically, if you have dates from the Jewish, Byzantine, Muslim or Roman calendars in Antiquity or the Middle Ages. There are unbelievably many things to consider. There's much more to it than considering the CE or BCE starting date of a specific calendar. Some calendars are actually shorter than ours. Some calendars actually have a fixed number of 30 days per month without any extra days or leap years, so they would be off by a few years by now. Then you have to consider that we have the leap years in our calendar, except every hundred years, we don't have a leap year where it should be, but every five hundred years, we do have one and stuff like that. But it gets even worse in the Christian Middle Ages, because people sometimes did not use the Julian or Gregorian calendars, but just wrote "two days after St James' day in the third year of our glorious King Richard II".
Really, dates in the Middle Ages are no fun at all.