Plutarch of Chaeronea (46-c.122): influential Greek philosopher and author, well known for his biographies and his moral treatises. His biography is here; the following fragment is from his Life of Julius Caesar.
One of the problems that Julius Caesar wanted to solve, was that of the calendar. The old republican calendar did have twelve months that were supposed to be more or less as long as the moon cycles. However, twelve lunar months correspond to 355 days; the deficiency was made up by the random additions of "intercalary" months. In Caesar's days, the calendar was seriously out of pace with the seasons. Following an advice of Cleopatra's court astronomer, he added 67 days to the year 45 BCE and introduced the modern European calendar with twelve months of 30 and 31 days. The reform is described by Plutarch of Chaeronea (46-c.120) in chapter 59 of his Life of Julius Caesar.
The translation below was made by Robin Seager.
[59.1] Caesar's reform of the calendar and the corrections made in the irregularity of reckoning time were not only studied by him with the greatest scientific skill, but were brought into effect and proved extremely useful.
[59.2] In very ancient times there had been great confusion among the Romans with regard to the relation of the lunar to the solar year, with the result that festivals and days of sacrifice gradually got out place and finally came to be celebrated at the very opposite seasons to what was originally intended.
[59.3] Nor was the confusion confined to the remote past. Even at this time most people were completely ignorant on these subjects; only the priests knew the proper time, and they, without giving any notice, would suddenly insert in the calendar the intercalary month known as Mercedonius.note[A Greek name. The Roman month was February.]
[59.4] It is said that this month was first put in by [the legendary] king Numa who thus managed to find an unsatisfactory and short-lived remedy for the error in the adjustment of the sidereal and solar cycles. I have dealt with this subject in my Life of Numa.
[59.5] Caesar, however, put the problem before the best scholars and mathematicians of the day and, out of the various methods of correction already in use he formed a new method of his own which was more accurate than any of them. It is he one still used by the Romans, and it seems that they, better than all other people, have avoided the errors arising from the inequality between the lunar and solar years.
[59.6] Yet even this gave offense to those who looked at Caesar with envious eyes and resented his power. Certainly Cicero, the orator, when someone remarked that the constellation Lyra would rise next day, remarked: "No doubt. It had been ordered to do so" - implying that even the rising of the stars was something that people had to accept under compulsion.