India's old literature starts from Ved. To study them it is necessary to take help from six Vedaang (parts of Ved): (1) Shikshaa (education), (2) Kalp, (3) Vyaakaran (grammar), (4) Nirukt, (5) Jyotish (astrology), and (6) Chhand (meters). Among the six, Jyotish is the most important . Bhaaskaraachaarya has said in "Siddhaant Shiromani", that if you take Ved as body then Vyaakaran is its mouth, Jyotish is its eyes, Nirukt is its ears, education is its nose and Chhand are its feet. Because of being eyes, Jyotish is at the highest, because a blind person cannot act properly in spite of having all other organs intact.

Jyotish is of two types - Theoretical (astronomy) Jyotish and Result Jyotish (astrology). In ancient Bhaarat (India), Jyotish meant only astronomy. Astrology was more popular in Greece, and it became popular in Bhaarat after the attack of Alexander.

There is one important book of Soorya Siddhaant in theoretical Jyotish books. Varaahmihiri has mentioned five books in "Panch-Siddhaantikaa" book and in Soorya Siddhaant's first chapter, the time measurement has been described in detail.

According to the speed of the Prithvi (Earth), Soorya (Sun), Chandra (Moon) and Nakshatra (stars), there are four types of days. By circling one Nakshatra, the next day is called "Nakshatra Ahoraatra (day+night)". From one sunrise to another sunrise, the period is called "Saavan Ahoraatra". From one Moon rise to another Moon rise, the period is called "Chandra Day" (Moon day, or Lunar day). And our Earth day ("Saur Day") depends on the annual speed of the Earth.

30 Nakshatra Ahoraatra make one "Nakshatra Month". 30 Saavan Ahoraatra make one Saavan Month, 30 Tithi (Lunar Day) make one Chandra Maas, and from One Sankraanti to another Sankraanti makes the Saur Maas (solar month).

12 Nakshatra Months make one Nakshatra Year, 12 Saavan Months make one Saavan Year, 12 Chandra Months make one Chandra year (Lunar Year) and 12 Saur Months make one Saur Year (Solar Year).

12 Saur months or 1 Saur Year = 1 Divine Ahoraatra (day+night)
(When Devtaa have their day, Asur have their night)

30 Divine Ahoraatra  =  1 Divine Month
12 Divine Months or 360 Saur Years   =  1 Divine Year

12,000 Divine Years or 4,320,000 Saur Years  =   1 4-Yug period or 1 Mahaa Yug
Since these 4 Yug are of different time span, that is why only Mahaa Yug is given here. One thing more: these years are in proportion of 4:3:2:1 years. Thus Sat Yug has 4,000 Divine Years, Tretaa Yug has 3,000 Divine years and so on. The 4 Yug and their time spans including their Sandhyaa and Sandhyaansh are as follows:

Sat / Krit Yug  =  4,000 + 800 Divine Years = 4,800 Divine Years  OR
                 4,800 Divine x 360 Saur Years = 1,728,000 Saur Years
Tretaa Yug      =  3,000 + 600 Divine Years = 3,600 Divine Years  OR
                 3,600 Divine x 360 Saur Years = 1,296,000 Saur Years
Dwaapar Yug    =  2,000 + 400 Divine Years = 2,400 Divine Years  OR
                 2,400 Divine x 360 Saur Years =    864,000 Saur Years
Kali Yug          =  1,000 + 200 Divine Years = 1,200 Divine Years  OR
                  1,200 Divine x 360 Saur Years =   432,000 Saur Years

Total Years in 1 Mahaa Yug (4-Yug period) = 12,000 Divine Years, OR
Total Saur Years in 1 Mahaa Yug = 12,000 Divine x 360 Saur Years = 4,320,000 Saur Years

Every Yug has Sandhi Kaal before and after, and they are of equal time. Thus Kali Yug is of 432,000 Saur Years, excluding 36,000 Saur years are in the beginning (Sandhi), and 36,000 Saur Years are in the end (Sandhyaa).

After Mahaa Yug

71 Mahaa Yug make 1 Manvantar
And every Manvantar also has Sandhi in the beginning and end. This Sandhi is equal to Sat Yug time. 

14 Manvantar+15 Manvantar Sandhi make up 1 Kalp 
So the time period of 1 Kalp is equal to 1,000 Mahaa Yug. Thus in 1 Kalp there are 44,320 million Saur Years, and this is Brahmaa's Ahoraatra (day+night).

1 Kalp  =  Brajmaa's 1 day+night
30 Brahmaa's Ahoraatra (Days) is Brahmaa's 1 Month, and 
12 Brahmaa's Months is Brahmaa's 1 Year, and
100 Brahmaa's Years is Brahmaa's age

Present Time

According to Soorya Siddhaant, which was written after Krit Yug of this Mahaa Yug. Till this time Brahmaa's half age (50 years) had gone by. And it is the first Kalp of His remaining half age. 6 Manvantar, along with Sandhi have passed and this is Kali Yug of 28th Mahaa Yug of the 7th Manvantar.

This Kali Yug was started in 3102 BC, therefore this year (2004) is 1,972,949,105th year of the present Kalp.