Most ancient civilizations had some sort of calendar, some very good ones. The early Romans had a calendar as well. But in Rome, politics began to affect the calendar.
Too funny (and typically Roman - very practical!) During the Roman Republic, if the priests did not like someone who had been elected to power, the priests removed months from the calendar, to shorten the year. Since people could only rule for a certain number of years, that shortened their term of office. But, if the priests liked the person in power, they added months to the calendar, so their favorite rulers had longer to rule. It was a crazy system, and pretty soon, the calendar was a mess.
When Julius Caesar came to power, he decided to throw out the old calendar and invent a new one. "His" calendar was remarkably like the ancient Egyptian calendar, but of course, like all things Roman, no credit was given to an invention by another civilization. The new calendar was Caesar's calendar. The new calendar had 365 days each year for three years, and 366 days on the fourth year, just like our calendar today. Julius Caesar named a month after himself, the month of July.
If you've ever wondered why February is the shortest month in our calendar, it all goes back to ancient Rome. When Augustus Caesar, Julius Caesar's adopted son, took over the government of ancient Rome, and called himself emperor (the first emperor of ancient Rome), he wanted a month named after himself, just as his father, Julius Caesar, had done. He took the month after July and renamed his month August. But August only had 30 days, not 31. An emperor could not have a short month. To fix that, the astronomers took a day from February and added it to August. That's why February is the shortest month of the year still today!