The very first formal calendar dates back to the Bronze Age. Since scribbling and documenting started during the ancient era, many forms of calendars were developed. The Sumerian calendar was the earliest, followed by the Egyptian, Assyrian and Elamite calendars.
In the ancient Sumerian calendars a year was divided into 12 lunar months of 29 or 30 days, where each month began with the sighting of a new moon.
Another popular, historical calendar was the Julian calendar, which was reformed by Julius Caesar in 46 BC.
A major calendar reform was led by Khayyam in Persia during the 11th century when, measuring the length of the year was prudently fashioned to 365.24219858156 days. Later, the first calendars based on Zoroastrian cosmology appeared somewhere between 650 to 330 BC.
The Gregorian calendar is the most widely used international calendar. It is a more clarified version of the Julian calendar.
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