Time as the one which is eternal, and indeed it is true on which our whole system of Universe works. This system according to our Sanatana Dharma is divided into Four Yugas and is mentioned in the Bhagvata Puran. The four Yugas are the Krita or the Satya Yuga, Treta Yuga, Dwapara Yuga, and the Kali Yuga. A Yuga is approximately divided into 4, 32,000 million years.
The nature of time is that it begins and ends in itself, but never stops. The moment it ends, it begins too. Hence, each Yuga is said to have decreased one fourth less than the preceding Yuga. This is to set right the balance of life created. To adjust the life cycles of life born in each Yuga, time compresses itself in each Yuga. A Yuga can be divided again into four quarters.
The Satya Yuga comprised of 1,728, 000 years. All the four quarters of the Yuga went in accordance to Dharma in this Yuga, and hence it is called the age of truth and perfection, and where humans are kind, helpful and righteous and long living. There was no disease bothering and the worlds were united in one Dharma. Great Bhgavattomas like Prahlad, Rishi Narada and Dhruva were born in this Yuga.
The Treta Yuga had a span of 1,296, 000 years. Treta meaning three signifies that there are three quarters which go in accordance in Dharma, while one goes with sins. This was the Yuga were great empires rose and the geography of land was divided. However, with wars and want for power, the virtues diminished and disease rose. But there were virtuous kings who showed the importance of Dharma, and it was in this very Yuga that Bhagwan Visnhu incarnated as Shri Ram. Humans life span descended to an average of 10, 000 years and the people were little less built than the Satya Yuga.
The Dwapara Yuga lasted for 864, 000 years, as it mentions, there were two quarters of time standing on Dharma, while the other two resorted to injustice. Humans fought with each for power and supremacy. The average life span of the humans decreased to a span of 1000 years. It required a strong force of Dharma to keep the humans united and hence was the avatar of Bhagwan Vishnu as Shri Krishna, who taught that Dharma is above everything and everyone else.
The Kali Yuga is said to be of 4, 32, 000 years, in which the present life exists. This is the last Yuga of the cycle and here, while one quarter rests on Dharma, the rest of the three are dominant with sins. This signifies the end of the time cycle and hence slowly things lead to destruction. Disease and sickness in dominant, nature is destroyed due to the selfish nature of humans and all knowledge is lost. The average life span of humans is 100 years. When sins reach the pinnacle, Bhagwan Vishnu incarnates in the form of Kalki avatar,destroys the degenerated life and clears the way for a new cycle of life to start and again giving rise to the Satya Yuga.
Time is the one which has seen the ages passing and adapting itself to the changes in a way which works for the betterment of life and Dharma in the world. It moves through different cycles, but never stops as it moves on Dharma which is the single and eternal factor keeping the entire Universe moving and working for a better life each day.