The Kali Yuga (Iron Age) our present age, was preceded by three others Yugas:
Satya or Krita Yuga (Golden Age),
Treta Yuga (Silver Age) and the
Dwapara Yuga (Bronze Age).
In the Mahabharata, Hanuman gives the following description of the Yuga Cycle to the Pandava prince Bhima:
"The Krita Yuga was so named because there was but one religion, and all men were saintly: therefore they were not required to perform religious ceremonies... Men neither bought nor sold; there were no poor and no rich; there was no need to labour, because all that men required was obtained by the power of will... The Krita Yuga was without disease; there was no lessening with the years; there was no hatred, or vanity, or evil thought whatsoever; no sorrow, no fear.
In the Treta Yugasacrifices began, and the World Soul became Red; virtue lessened a quarter. Mankind sought truth and performed religious ceremonies; they obtained what they desired by giving and by doing.
In the Dwapara Yuga the aspect of the World Soul was Yellow: religion lessened one-half. The Veda was divided into four parts, and although some had knowledge of the four Vedas, others knew but three or one. Mind lessened, Truth declined, and there came desire and diseases and calamities; because of these men had to undergo penances. It was a decadent Age by reason of the prevalence of sin. (Twilight of Dvapara was during the Age of Taurus)
"World Soul" is Black in hue; only one quarter of virtue remains, which slowly dwindles to zero at the end of the Kali Yuga.
Men turn to wickedness; disease, lethargy, anger, natural calamities, anguish and fear of scarcity dominate.
Penance, sacrifices and religious observances fall into disuse.
All creatures degenerate. Change passes over all things, without exception. (Kali Yuga proper began in Age of Aries)
NOTE: The original Yuga Cycle was based on the Saptarsi Calendar. It was of 12,000 years duration, comprised of four Yugas of equal duration of 2,700 years each, separated by transitional periods of 300 years. The complete Yuga Cycle of 24,000 years was comprised of an ascending and descending Yuga cycle, which followed each other for eternity like the cycles of day and night.
For the past 2,700 years we have been evolving through the ascending Kali Yuga, and this Yuga is coming to an end in 2025.
The end of the Yuga will inevitably be followed by cataclysmic earth changes and civilization collapses, as is characteristic of the transitional periods
Another calculation (by hand) gives theend of the Twilight Period of the Kali Yuga as 2,442 AD and,