Conversion between Julian and Gregorian Calendars. Currently, the Julian calendar is 13 days behind the Gregorian calendar. So, to convert from the Julian calendar to the Gregorian calendar, add 13 days; to convert in the opposite direction, subtract 13 days. The gap between the two calendar systems will increase to 14 days in the year 2100.
Julius Caesar thought it would be appropriate for January, Janus's namesake month, to be the doorway to a new year, and when he created the Julian calendar, he made January 1 the first day of Orthodox New Year, also known as the Old New Year, marks the start of the new year according to the Julian calendar. The Julian Calendar was off by 11 minutes every solar year, which added up and made us lose 10 days by the year 1582. The Gregorian Calendar is much more accurate and is only off by 26 seconds every solar year, which will eventually add up to make us lose 1 day by the year 4909.The Julian calendar has two types of year: "normal" years of 365 days and "leap" years of 366 days. There is a simple cycle of three "normal" years followed by a leap year and this pattern repeats forever without exception. The Julian year is, therefore, on average 365.25 days long.OMWVP.