Julius Caesar and Leap Days . ... In 46 BC, Julius Caesar , pictured above in a self-decreed minted coin, created a calendar system that added one leap day every four years. ... By adopting a leap year with an extra day every four years, the calendar year would drift much less. This Julian Calendar system was used until the year 1582 when Pope Gregory XIII provided further fine-tuning when he added that leap days should not occur in years ending in "OO", unless divisible by 400. ...