In Vedic times "Thrudi" is considered as the basic unit of time. When 100 lotus petals are kept one on other together and a sharp needle is passed through them, the time taken by the needle to pass through one petal is called as one "Thrudi".
100 Thrudi = 1 Lavam
30 Lavam = 1 Nimisham (Moment)
18 Nimisham = 1 Kashta
30 Kashta = 1 Kala
30 Kala = 1 Khadika
2 Khadika = 1 Muhurtham
30 Muhurtham= 1 day
10 Gurvaksharam = 1 Asu (Pranan)
6 Asu = 1 Vinazhika
60 Vinazhika = 1 Nazhika
60 Nazhika = 1 day
Modern time measurement:
24 Minute = 1 Nazhika
2 1/2 Nazhika = 1 Hour
3 Hour = 1 Yamam
24 Hour = 1 Day
7 day = 1 Week
15 day = 1 Paksham
30 day = 1 Month
2 Month = 1 Rithu
3 Rithu = 1 Ayanam
2 Ayanam = 1 Year
Calculation of Yugas:
There are 4 Yugas named Krita Yuga, Thretha Yuga, Dwapara Yuga & Kali Yuga
Kritha Yuga contains 17, 28,000 years and the present Kritha Yuga started in the month of Vrichika (November) on a Wednesday and Shuklapaksha Navami.
Tretha Yuga contains 12, 96,000 years and the present Tretha Yuga started in the month of Idava (May) on a Monday and Shuklapaksha Trithiya.
Dwapara Yuga contains 8, 64,000 years and the present Dwapara Yuga started in the month of Kumbha (February) on a Friday and Amavasi.
Kali Yuga contains 4, 32,000 years and the present Kali Yuga started in the month of Kanni (September) on a Sunday and Krishnapaksha Thrayodashi. It is told that when this Kali Yuga started all seven planets were in in one Rasi (House).
Total of these four Yugas is called as one Chathuryuga. One Chaturyuga contains 43,20,000 years. It is believed that Dharma will diminish in each Yuga like be 100% Dharma in the first Yuga, 75% Dharma in the second Yuga, 50% Dharma in the third Yuga and 25% Dharma in the present Kali Yuga.
71 Chathuryuga = 1 Manwantharam
14 Manwantharam = 1 Kalpa
2 Kalpas will be Lord Brahma's one day. One Brahma year contain 360 Brahma days. Each Brahma have longevity of 100 Brahma years(72,000 Kalpa).
We are located in the fifty-first Brahma year of the life of our Brahma.
Within that Brahma year, we are in the first Brahma day, called the Varaha kalpa.
Six Manwanthram are completed in the present Kalpa. Swyambhuv, Sarochisham, Uttama, Thamasa, Raivatam and Chakshusham are those six Manwantharam. Vaivaswatha is the present Manwantharam. 27 Chathuryuga were finished in this Manwantharam. Now 5095 years were finished in Kali Yuga of this 28th Chathuryuga of the present Manwantharam.
Following are the details of 14 Manwantaras in the present Kalpa.
Each Manwantara is ruled over by a Manu and there are fourteen Manwantaras in any kalpa. The gods (devas), the seven great sages (saptarshis), and the Indra, change from one Manwantara to another.
The first Manu was Svayambhuva. The names of the gods then were yama and the names of the seven sages were Marichi, Atri, Angira, Pulastya, Pulaha, Kratu and Vashishtha.
The second Manu was Svarochisha. The names of the gods then were tushita and the names of the seven sages were Agnidhra, Agnivaha, Medha, Medhatithi, Vasu, Jyotisvana and Dyutimana.
The third Manu was Outtama. The names of the gods then were rishabha and the names of the seven sages were the urjjas. (The individual names of the sages are not given).
The fourth Manu was Tamasa. The names of the gods then were satya and the names of the seven sages were Gargya, Prithu, Agni, Janya, Dhata, Kapinka and Kapivana.
The fifth manu was Raivata. The names of the gods then were raibhya and the names of the seven sages were Vedavahu, Jaya, Muni, Vedashira, Hiranyaroma, Parjanya and Urddhavahu.
The sixth Manu was Chakshusha. There were five types of gods in the sixth Manwantara and their names were adya, prasuta, ribhu, prithugra and lekha. The names of the seven sages were Bhrigu, Naha, Vivasvana, Sudharma, Viraja, Atinama, and Asashishnu.
The seventh Manwantara is the manvanatara that is now going on. The seventh Manu is Vaivasvata. The names of the seven sages are Atri, Vashishtha, Bhavya, Kashyapa, Goutama, Bharadvaja and Vishvamitra. The gods now are forty-nine vayus, eleven rudras, two ashvinis, twelve adityas and eight vasus.
The remaining seven Manwantaras will come in the future.
The eight Manu will be Savarni. The Shiva Purana gets extemely confused here and it is not possible to make out clearly who the gods will be in the future Manwantaras. But the names of the seven sages of the eighth Manwantara are Viravana, Avanivana, Sumantra, Dhritimana, Vasu, Varishnu and Arya.
The nine Manu will be Rohita. The names of the seven sages will be Medhatithi, Vasu, Bhargava, Angira, Savana, Havya and Poulaha.
The tenth Manu will be Merusavarni. The names of the gods then will be dvishimanta and the names of the seven sages will be Havishmana. Pulaha, Sukriti, Ayomukti, Vashishtha, Prayati and Nabhara.
The eleventh Manu will be Brahmasavarni. The names of the seven sages will be Havishmana, Kashyapa, Vapushmana, Varuna, Atreya, Anagha and Angira.
The twelfth Manu will be Dharmasvarni. The names of the sages will be Dyuti, Atreye, Angira, Tapasvai, Kashyapa, Taposhana and Taporati.
The thirteenth Manu will be Rouchya. The names of the seven sages will be Kashyapa, Magadha, Ativahya, Angirasa, Atreya, Vashishtha and Ajita.
The fourteenth and last Manu will be Bhoutya. The names of the seven sages are not known.
End of each Kali Yuga(end of each Chaturyuga/Mahayuaga) various calamaties cause a good deal of destruction.
At the end of Brahma's daytime period, the three upper worlds (Bhuloka, Bhuvarloka, Swarloka) and the seven underworlds (of the nagas) are temporarily dissolved(pralaya); that is, the same folks can be reincarnated when the next day(Kalpa) of Brahma begins.
The Vishnu Purana states that at the end of the daytime period of Brahma, a dreadful drought occurs that lasts 100 years, and all the waters are dried up. The sun changes into seven suns, and the three worlds (Bhurloka or Earth, Bhuvarloka or the lowest heaven, and Svarloka or the next higher heaven) and the underworlds are burned bare of life. The inhabitants of Bhuvarloka and Svarkloka flee to the next higher heaven, Maharloka, to escape the heat; and then to the next higher heaven, Jnanaloka.
Then mighty clouds form and the three worlds are completely flooded with water. The lord Vishnu reposes on the waters in meditative rest for another whole kalpa (4.32 billion years) before renewing the creation.
The destruction that takes place at the end of a daytime of Brahma is referred to as naimittika, which is incidental or occasional. The characteristic of this destruction is that the three worlds continue to exist but are made uninhabitable. The souls of individuals also continue to exist to be reincarnated in the next daytime of Brahma.
The lifespan of Brahma is 100 Brahma years, or 72,000 kalpas, or 311.04 trillion human years.
At the end of the life of Brahma, all worlds are completely dissolved (mahapralaya). No one is reincarnated from these worlds ever again. After that new Lord Brahma will come to do the creation.