In ancient Rome, the calendar consisted of 12 months, lasting a total of 355 days with an additional month added during leap years. The figuring out of the exact days was quite confusing, and Julius Caesar introduced a reform that was adopted in the year 46 b.c.e. This Julian calendar had a year of 12 months lasting 365 days, with a single leap day added every 4 years. This simpler calendar became the most popularly used calendar in most of Europe, as well as in some other locations, including settlements in the Americas. 

Unfortunately, it was off by more than 10 minutes per year, and after hundreds of years, it was off by several days. This was corrected in 1582 c.e. when Pope Gregory XIII instituted the Gregorian calendar, which has become the most accepted calendar in the world (although it wasn't adopted in Greece until 1923).