The calendar used in western civilization in fact is the Egyptian calendar with its 365 days in a year. The Egyptian calendar was introduced in an improved form by Julius Caesar in Rome in 46 b.C. Since a minor correction was added under Pope Gregory XIII in 1582, we now speak of the Gregorian calendar. Which in fact is too small an honour to the Egyptians...
The following is an adoption from http://serendipity.magnet.ch/hermetic/cal_stud/cal_art.htm:
Following his conquest of Egypt in 48 B.C. Julius Caesar consulted the Alexandrian astronomer Sosigenes about calendar reform. The calendar which Julius Caesar adopted in 46 BC was identical to the Alexandrian Aristarchus' calendar of 239 BC, and consisted of a solar year of twelve months and of 365 days with an extra day every fourth year. It is unclear as to where or how Aristarchus arrived at this calendar, but one may speculate that Babylonian science was involved.