The Twelve Caesars - page 31, Julius Caesar

Caesar next turned his attention to domestic reforms. First he reorganized the Calendar which the College of Priests had allowed to fall into such disorder, by inserting days or months as it suited them, that the harvest and vintage festivals no longer corresponded with the appropriate seasons. He linked the year to the course of the sun by lengthening it to 365 days (the year had previously consisted of 355 days), abolishing the short extra month and adding an entire day every fourth year. But to make the next first of January fall at the right season, he drew out this particular year (46 B.C.) by two extra months, inserted between November and December, so that it consisted of fifteen, including the intercalary one inserted after February in the old style.


Julius Caesar's calendar reforms became the standard for much of the West for at least 1600 years in some places (mostly Catholic countries that adopted the Gregorian calendar, which was a modernized version of the Julian calendar) and well into the 20th Century in others. This calendar and its successor then spread into worldwide usage, and it has led to the way we track the passage of time even today. It's not an overstatement to say that this one act by a power-hungry, cunning, swiving, Roman general who named himself "dictator in perpetuity" has had a greater impact on civilization than any other effort.