Manvantara or Manuvantara or "Manvanter" , or age of a Manu, the Hindu progenitor of humanity, is an astronomical period of time measurement. Manvantara is a Sanskrit word, a compound of manu and antara, manu-antara or manvantara, literally meaning the duration of a Manu, or his life span.
Each Manvantara is created and ruled by a specific Manu, who in turn is created by Brahma, the Creator himself. Manu creates the world, and all its species during that period of time, each Manvantara lasts the lifetime of a Manu, upon whose death, Brahma creates another Manu to continue the cycle of Creation or Shristi, Vishnu on his part takes a new Avatar, and also a new Indra and Saptarishis are appointed.
Fourteen Manus and their respective Manvantaras constitute one Kalpa, Aeon, or a ‘Day of Brahma’, according to the Hindu Time Cycles and also the Vedic timeline. Thereafter, at the end of each Kalpa, there is a period - same as Kalpa - of dissolution or Pralaya, wherein the world (earth and all life forms, but not the entire universe itself) is destroyed and lies in a state of rest, which is called the, ‘Night of Brahma’.
After that the creator, Brahma starts his cycle of creation all over again, in an endless cycle of creation followed by Absorption for which Shiva, Hindu God of Absorption, and also renewal, is invoked towards the end of each such cycle.
The 14 appointed Indras for each kalpa are: Visvabhuk, vipascit, sukirti, sibi, vibhu, manobhuva, ojasvin, the powerful bali, adbhuta, santi, ramya, devavara, vrsa rtadhaman, divassvamin and suci. These are the fourteen sakras(indras).
The actual duration of a Manvantara, according to the Vishnu Purana is seventy one times the number of years contained in the four Yugas, with some additional years, adding up to 852,000 divine years, or 306,720,000 human years.  Seven Rishis, certain (secondary) divinities, Indra, Manu, the king and his sons, are created and perish in one interval (called a Manvantara) equal to seventy-one times the number of years contained in the four Yugas, with some additional years: this is the duration of the Manu, the (attendant) divinities, and the rest, which is equal to 852,000 divine years, or to 306,720,000 years of mortals, independent of the additional period. Fourteen times this period constitutes a Bráhma day, that is, a day of Brahmá; the term (Bráhma) being the derivative form. The Brahma life span is 100 Brahma varshas. The following table will illustrate clearly the link to our years and Brahma years.
(12,000 Deva Vatsaras are defined as, 4,800 Deva Vatsaras of satya yuga, 3,600 Deva Vatsaras of Treta Yuga, 2,400 Deva Vatsaras of Dvapara Yuga and 1,200 Deva Vatsaras of Kali Yuga (which is 1,200 * 360 = 432,000 human years); summing up to 12000)
2 Kalpas = 1 day + 1 Brahma ratra
360 days of Brahma = 1 Brahma varsha
Modern scientific astronomy estimates the Age of the Universe as around 13 Billion years (13 * 109 years). Conversion of 1 day of Brahma into human years yields 8.58816 * 109 years (derived as 2 kalpas * 14 Manvantaras * 71 Chaturyugas * 12,000 Deva vatsaras * 360 human years). According to Vedas, there are 504 000 Manus manifested during the lifetime of one Brahmā (311,040,000,000,000 human Earthly years), 5040 Manus in one year of Brahma, and 420 Manus in one month of Brahma. (See for more details: List of numbers in Hindu scriptures.)