By the time he reformed the Julian calendar in using the observations of Christopher Clavius and Johannes Kepler , it had drifted 10 days off course. To this day, most of the world uses his Gregorian calendar. Ironically, by the time the Catholic church buckled under the weight of the scientific reasoning that pointed out the error, it had lost much of its power to implement the fix. The "new" calendar, as we know it today, was not adopted uniformly across Europe until well into the 18th century. In some ways, yes.
The Roman calendar
You may know that the Ides of March -- the day on which Julius Caesar was assassinated -- was the 15th of March, but that doesn't mean the Ides of a month was necessarily on the 15th. The Roman calendar was originally based on the first three phases of the moon, with days counted, not according to a concept of a week, but backward from lunar phases. The new moon was the day of the Kalends, the moon's first quarter was the day of the Nones, and the Ides fell on the day of the full moon. The Kalends' section of the month was the longest, since it spanned two lunar phases, from the full to the new moon.
GODOT – Graph of Dated Objects and Texts
Conversion Converting dates in the calendar we use into Roman dates is tricky and involves some degree of compromise. The Roman calendar was altered many times as errors in previous calendars were corrected and political considerations led to compromises in those changes. So whether it is the day, the month or the year we convert into 'Roman' the final result may end up overall as something a Roman would not recognise.
The Romans borrowed parts of their earliest known calendar from the Greeks. The calendar consisted of 10 months in a year of days. The Romans seem to have ignored the remaining 61 days, which fell in the middle of winter. The last six names were taken from the words for five, six, seven, eight, nine, and ten.