Prior to the creation of the universe. Lord Vishnu lies asleep on the ocean of all causes. He rests upon a serpent bed with thousands of cobra-like hoods. While asleep, a lotus sprouts from His navel. Upon this lotus is born Brahma the creator of the universe. Lord Brahma lives for a hundred years and then dies, while Lord Vishnu remains. One year of Brahma consists of three hundred and sixty days. At the beginning of each day Brahma creates the living beings that reside in the universe and at the end of each day the living beings are absorbed into Brahma while he sleeps on the lotus. On day of Brahma is known as a KALPA. Within each KALPA there are fourteen MANUS and within each MANU are seventy one CHATUR-YUGAS. Each CHATUR-YUGA is divided into four parts called YUGAPADAS. 

From the first chapter of Surya-Siddhanta, the most revered authoritative source of Hindu astronomy, we have the following passage: 

11. That which begins with respirations (prana) is called real.......Six respirations make a vinadi, sixty of these a nadi: 

12. And sixty nadis make a sidereal day and night. Of thirty of these sidereal days is composed a month; a civil (savana) month consists of as many sunrises; 

13. A lunar month, of as many lunar days (tithi); a solar (saura) month is determined by the entrance of the Sun into a sign of the zodiac; twelvemonths make a year. This is called a day of the gods. 

14. The day and night of the gods and of the demons are mutually opposed to one another. Six times sixty of them are a year of the gods, and likewise to the demons. 

15 & 16. Twelve thousand of these divine years are denominated a chatur-yuga; of ten-thousand times four hundred and thirty two solar years is composed that chatur-yuga, with its dawn and twilight. The difference of the krita-yuga and the other yugas, as measured by the difference in the number of the feet of virtue in each is as follows: 

17. The tenth part of a chatur-yuga, multiplied successively by four, three, two, and one, gives the length of the krita and the other yugas: the sixth part of each belongs to its dawn and twilight. 

18. One and seventy chatur-yugas make a manu; at its end is a twilight which has the number of years of a krita-yuga, and which is a deluge. 

19. In a kalpa are reckoned fourteen manus with their respective twilights; at the commencement of the kalpa is a fifteenth dawn, having the length of a krita-yuga. 

20. The kalpa, thus composed of a thousand chatur-yugas, and which brings about the destruction of all that exists, is a day of Brahma; his night is of the same length. 

21. His extreme age is a hundred, according to this valuation of a day and a night. The half of his life is past; of the remainder, this is the firsts kalpa. 

22. And of this kalpa, six manus are past, with theirrespective twilights; and of the Manu son of Vivasvat, twenty seven chatur-yugas are past; 

23. Of the present, the twenty eighth chatur-yuga, this krita yuga is past........ 

Now to make plain what is stated above. Commentaries are very clear on the fact that in verse 12 the "sidereal day" refers to a revolution of the Earth relative to any fixed star and is the true revolution reference point of the Earth. Verse 13 refers to "a day of the gods" means one sidereal year. A night of the gods is half a sidereal year. Verse 21 mentions "his extreme age is a hundred refers to the lifespan of Brahma and consists of one hundred years of 360 days. Each of these days being two kalpas long. Verse 23 shows that the Surya-Siddhanta was composed right after krita-yuga and during the treta-yuga. The present yuga we are in right now is the kali-yuga which is said to have begun on Friday February 18th 3102 B.C. of the Julian calendar. This becomes clearer when represented in a tabular form.