Time calculations from the smallest unit up to the cosmic scale.
[Based on the Third Canto, Chapter 11 of the  Srimad-Bhagavatam as explained by His Divine Grace A. C. Bhaktivedanta Swami Prabhupada.]
The atomic description of the Srimad-Bhagavatam is almost the same as that of modern science. This is further described in the Paramanu-vada of Kanada. Time is measured in terms of its covering a certain space of atoms. Standard time is calculated in terms of the movement of the sun. The time covered by the sun in passing over an atom is calculated as atomic time.
The division of gross time is calculated as follows: two atoms make one double atom, and three double atoms make one hexatom. The atom is described as an invisible particle, but when six such atoms combine together, they are called a trasarenu. The time duration needed for the integration of three trasarenus is called a truti, and one hundred trutis make one vedha. Three vedhas make one lava. It is calculated that if a second is divided into 1687.5 parts, each part is the duration of a truti, which is the time occupied in the integration of eighteen atomic particles.
Two paksas comprise one month, and twelve months comprise one calendar year, or one full orbit of the sun. A human being is expected to live up to one hundred years. 
During the period of one month the moon wanes and is called krsna-paksa, the dark moon or amavasya. In the same month the moon waxes and is called gaura-paksa or sukla-paksa, the full moon or purnima. Thus purnima to amavasya is called krsna-paksa (dark moon) and amavasya to purnima is called sukla-paksa (bright moon). Two months equal one season. During the first six months the sun travels from south to north (uttarayana). During the second six months the sun travels from north to south (daksinayana). Two solar movements equal one day and night of the demigods.
The Four Cosmic Ages (Yugas)
When explaining the various measurements of time, Visnu Purana (1.3), Srimad-Bhagavatam (3.11.18-39), Bhagavad-gita (8.17), Vayu Purana (chapter 57) and others, such as the Mahabharata (Santi-parva, 231.12-20), all agree on the measurements of the durations of the yugas, as explained below.
One cycle of the four yugas together is 12,000 years of the devas called divine years. Each of these years is composed of 360 days, and each of their days is equal to one human year. So Krta-yuga, or Satya-yuga, is 4000 divine years in length, Treta-yuga is 3000 divine years in length, Dvapara-yuga is 2000 divine years in length, and Kali-yuga is 1000 divine years long, with the addition of the conjoining portions of the sandhya and sandhya-sandhyamsa. Each yuga is preceded by a period called a sandhya, which is as many hundred years in length as there are thousands of years in that particular yuga. Each yuga is also followed by a period of time known as a sandhyamsa of the same length. In between these periods of time is the actual yuga.
Time is the potency of the almighty Personality of Godhead, Hari, who controls all physical movement although He is not visible in the physical world. The complete calculation of the time of creation, maintenance and dissolution, measured in terms of the circulation of the total planetary systems until the end of creation, is known as the supreme kala.
In the Brahma-samhita it is stated that the sun is the eye of the Supreme and it rotates in its particular orbit of time. Similarly, beginning from the sun down to the atom, all bodies are under the influence of the kala-cakra, or the orbit of eternal time, and each of them has a scheduled orbital time of one samvatsara