Kalachakra is the Hindu "wheel of time." According to tradition, all sentient beings are destined to pass through the four great epochs in every cycle of creation and destruction. This cycle turns full-circle at the end of kalpa. A kalpa is a period of 4,320,000,000 years, and is divided into four epochs or yugas. The four great epochs are: Satya-Yuga, Treta-Yuga, Dwapar-Yuga and Kali-Yuga. Satya-Yuga, the Age of Truth, is said to last for four thousand divine years, Treta-Yuga for three thousand, Dwapara-Yuga for two thousand and Kali-Yuga will last for one thousand divine years.
The four epochs represent periods of progressive decay for man. The human lifespan, set at 100,000 in Satya-Yuga, is diminished in each yuga by a factor of ten reaching 100 years in Kali-Yuga. Also, each epoch also marks a reduction in the creative and productive energy of the Earth. Food thus begins to lose its powers of sustenance. People of Kali-Yuga are therefore lack the mental, physical, moral and spiritual strength of their forefathers.
At the onset of the present epoch, Kali-Yuga, the sage Vyasa felt the impending doom of the age to come, and prophesied the following: "Vyasa sees the effects of chronic malnutrition on generation after generation; he watches it gradually diminish their span of life along with their brain power; no one can escape the progressive drop in intelligence and ability to remember."
Thus Vyasa foretold that turbulent times would cause the moral and spiritual decline of an ignorant populace to hasten. Increasingly degraded generations, conceived in ignorance and growing up wild, swarm over the earth, which is now covered in Iron. The people allow leadership to fall into the hands of fiends and atrocities flourish under guise of 'high ideals.'
The Vedic date of advent of Kali-Yuga (c. 3000 BC) corresponds closely to the date set by modern historians for the rise of civilized life, an event signalled by the appearance of literacy and the emergence of complex urban societies. Also, the decline of man outlined in the Kalachakra closely resembles the Greek idea of the decline of man through the fours Ages of Gold, Silver, Copper, and Iron, as outlined by the poet Hesiod.