Wheels Within Wheels | The Vedic Concept Of Time

"Outside of the three planetary systems, the four Yugas multiplied
by one thousand comprise one day on the planet of Brahma. A similar
period comprises a night of Brahma, in which the creator of the
universe goes to sleep."
Srimad Bhagavatam, Canto 3, Chapter 11, Text 22

The Vedas and the Puranas describe a number of time cycles within cycles, from Paramanu (about 17 microseconds) to Maha-Manvantara (311.04 trillion years). The creation and destruction of the universe is a cyclic process, which repeats itself forever. Each cycle starts with the birth and lifetime of the Universe equaling 311.04 trillion years, followed by its complete annihilation which also prevails for the same duration (HERE).  

The universe always existed. It was not created, and the concept of eternal and cyclical time lies at the heart of the Hindu philosophy. In the Hindu conception of time there is no final cataclysm. The closing of one door implies the opening of another. 

Michael A. Cremo (1996): "Linear-progressivist time concepts [...]
pose a substantial barrier to truly objective evaluation of the
archeological record and to rational theory- building in the
area of human origins and antiquity." (HERE+ HERE)

Destruction of the cosmos only portends its re-creation. The entire material world is thus subject to everlasting cycles of creation, sustenance and destruction. This idea is closely related to the concept of Atman (the eternal self), ever-existing, not only in the future but also from the past. This notion of two-way eternity, however, is not reserved solely for the realm of spirit (Brahman) but extends to this temporal world. This universe exists for the lifetime of Brahma, the creator. His one day is 1,000 Maha-Yugas (Great Ages). The Puranic concept of time involves cycles of Yugas (Ages), each progressively shorter and more degraded.  

Each Yuga cycle is composed of four Yugas. The first, Satya-Yuga, is the Golden age and lasts 1,728,000 years. The second is the Silver Age, the Treta-Yuga, which lasts 1,296,000 years. The third is the Bronze Age, the Dvapara-Yuga, which lasts for 864,000 years. And the fourth is the Iron Age, the Kali-Yuga, lasting for 432,000 years. This gives a total of 4.32 million years for the entire Maha-Yuga cycle, the period of a Great Year. 

One thousand of such cycles 4.32 billion years make up one Day of Brahma, the demigod who governs the universe. One Day of Brahma is called a Kalpa. Each of Brahma's Nights lasts as long as his day. Life is manifest on earth only during the day of Brahma. With the onset of Brahma's night, the entire universe is devastated and plunged into darkness. When another day of Brahma begins, life again becomes manifest. Each Day of Brahma is divided into 14 Manvantara periods, each lasting 71 Yuga cycles. Preceding the first and following each Manvantara period is a juncture (sandhya). Typically, each Manvantara period ends with a partial devastation. 

Today we are living in the 28th Yuga cycle of the 7th Manvantara period of the present Day of Brahma. This would give the inhabited earth an age of 2.3 billion years. Altogether, 453 Yuga cycles have elapsed since this Day of Brahma began. Each Yuga cycle involves a progression from a golden age of peace and spiritual progress to a final age of violence and spiritual degradation. At the end of each Kali-Yuga, the earth is practically depopulated.  

Currently, 50 years of Brahma have elapsed. The last kalpa at the end of 50th year is called padma kalpa. We are currently in the first 'day' of the 51st year. This Brahma's Day, Kalpa, is named as Shveta-Varaha Kalpa. Within this Day, 6 Manvantaras have already elapsed and this is the 7th Manvantara, named as – Vaivasvatha Manvantara. Within this Vaivasvatha Manvantara, 27 Maha-Yugas, and the Krita, Treta and Dwapara Yugas of the 28th Maha-Yuga have elapsed. 

The current Kali Yuga began at midnight of February 17th to February 18th in 3102 BCE in the proleptic Julian calendar. As per the information above about Yuga periods, only 5,118 years are passed out of 432,000 years of current Kali Yuga, and hence another 426,882 years are left to complete this 28th Kali Yuga of Vaivaswatha Manvantara.The time elapsed since the current Brahma has taken over the task of creation can be calculated as

432,000 × 10 × 1000 × 2 = 8.64 billion years (2 Kalpa - Day and Night)
8.64 × 109 × 30 × 12 = 3.1104 Trillion Years (1 Year of Brahma)
3.1104 × 1,012 × 50 = 155.52 Trillion years (50 Years of Brahma)
(6 × 71 × 4,320,000) + 7 × 1.728 × 10^6 = 1,852,416,000 years elapsed in the first six Manvataras, and Sandhi Kalas in the current Kalpa
27 × 4,320,000 = 116,640,000 years elapsed in first 27 Maha-Yugas of the current Manvantara
1.728 × 10^6 + 1.296 × 10^6 + 864,000 = 3,888,000 years elapsed in the current Mahayuga
3,102 + 2,016 = 5,118 years elapsed in the current Kali Yuga.

So the total time elapsed since current Brahma is: 155,520,000,000,000 + 1,852,416,000 + 116,640,000 + 3,888,000 + 5,115 = 155,521,972,949,117 years (one hundred fifty-five trillion, five hundred twenty-one billion, nine hundred seventy-two million, nine hundred forty-nine thousand, one hundred seventeen years) as of 2016 A.D.

The main time cycle governing mankind is the precessional cycle:
The period of 25,000 years the seasons in the life of humanity.

Surya Siddhanta: "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; twelve months make a year. This is
called a day of the gods."