The Julian calendar came about in 46 B.C.E. when Julius Caesar had the old Roman calendar reworked to improve its accuracy. While it did prove to be more accurate, it still was off by a factor of some three days every 400 years in relation to observed seasons. Pope Gregory instituted a new reform in 1582 to correct the discrepancy, hence the Gregorian Calendar. Today the Gregorian is the international standard.
The difference, timewise, is about 13 days currently.