How The Calendar Was Made. About 3000 bce, one of the oldest calendars ever discovered was developed in ancient egypt by the nile river ’s yearly floods. Every four years, leap days are added to achieve this.
The calendar with a leap day was finally adopted by the romans under julius caesar in 46 bce with the first leap year added in 45 bce. Throughout history, various civilizations have developed their own calendars, based on different methods of measuring time.