Calendar is a term derived from the Latin calendarium (account book). The Romans called the first day of each month Kalendae, or calends. Debts were due on this day, so books to track payments were called calendarium, which is the origin of the modern calendar.
All calendars are based on recording time by using natural cycles: days, lunar cycles (months), and solar cycles (years). The year is 365 days 5 hours 48 minutes 46 seconds long or 365.242199 days. The time between full moons is 29.53 days.
The 12 month calendar dates back to the ancient Egyptians, who created a calendar of 12 months, each consisting of 30 days, comprising a year. They later added 5 days at the end of the year to roughly syncronize with the solar year.
However, since this calendar didn't quite align with a actual year, Greek rulers of Egypt under Ptolemy added the concept of a leap year, adding a day every 4 years.
