Short answer: Nippur, Sumeria circa 4,400 BC much longer answer: Perhaps the earliest known calendar was devised in the Sumerian city of Nippur, circa 11,000 BC. The interesting thing about this calendar is that it is quite similar to our modern calendar, using 365 days for a solar year, and dividing the year into twelve months, based loosely on the moon's revolutions. Not to mention, it was devised thirteen thousand years ago. This calendar system was preserved through the Babylonyans, Akkadians, Greeks, and Hebrews to become the system we use today. Proof of the date comes from a phenomenon known as the recession of the equinox. On the spring equinox, the sun moves through one of the zodiac constellations (Pisces, modern day). Every 2,160 years, that point passes into the previous sign, due to the collective action of the Earth's axial tilt. The next shift occurs in A Sumerian tablet in the Berlin Museum (VAT.7847) begins the list of constellations with Leo, which conclusively places the first calendar in the same times as the first farming, thirteen thousand years ago. Other, later, tablets continue the observance of the recession. This also proves that accurate astronomical data was collected, stored, and disseminated by a 'primitive' civilization without even a telescope. Even more curious, the ancient Sumerians knew of the outer planets (Uranus, Neptune, Saturn, and Pluto) more than ten thousand years before we did. Without the benefit of instruments. This early calendar underwent several minor revisions over the centuries, until the one which most closely parallels ours was devised (again, in Nippur) circa 4,400 BC. It's accuracy was the result of neary seven thousand years of precise astronomical observations and records. Hope that helps... Sources: Professor Stephen Langdon - "Tablets from the Archives of Drehem" Professor Jeremias - "The Old Testament in the Light of the Ancient East" Zecharia Sitchin - "The 12th planet"

Archaeologists have unearthed notched bones that are at least 20,000 years old. They think these were used to mark the passage of time long before early man had developed written language. In my opinion, notched bones do not a calendar make, but it does show that man was interested in the passage of time. Most likely the calendar would have been one of the first uses of writing when it was invented. Click here for more information on the naming of months and days etc. http://users.commspeed.net/k6xf/calendar.htm#ORG

Long time ago. 44 hundred years BC

As Zero stated, so far we have officialy traced it back to the ancient Summerians. But as we do more arcaelogical research, I am sure it will easily go farther back.