Planet Earth
Before we venture into the unknown, lets starts with things that science has uncovered and proven with enough evidence. Just like the existence of sun, we are quite sure about the earth rotating its own axis every 24 hrs and moon revolving around the earth.

Moon also revolves around its own axis and it takes the same time to revolve around the earth. This is known as synchronous orbit.

Solar System
Similarly, earth revolves around the sun and each orbit is observed as completion of a year. Sun also completes one revolution around its own axis every 25 days (24.47 days to be close to precision). The distance between center of earth and center of sun is 108 times the diameter of the sun. same is the case with earth and moon.

Now we are venturing into the zone that has no scientific proof yet. But, its logical as it follows the same principles of the so-far known celestial objects, though this concept was penned down at least 5000 years ago in the period of 3000 BC in India. Majority of this comes from Sage Ganita, mentioned in Mahabaratha.

Before we venture into the details, lets get an understanding of these measuring units in the units of our times.

deva (devi femminine) are the deities, can be inferred as super-humans. One full day (and night) of the devas (celestial) is called ayana. daytime is called utarayana is the first half of the year when it appears the sun is moving towards north and the night time is called dakshinayana, the later half of the year, when the sun appears to move towards south.
  • 1 celestial day      = 1 human year
  • 1 celestial month  = 30 celestial days = 30 human years
  • 1 celestial year     = 12 celestial months = 360 human years
Our Solar System is allegedly orbiting the galactic center and each orbit is known as maha yuga (or divya yuga). Maha yuga is a cycle of four yugas, namely, krita, treta, dwapara, kali. These yugas are decided based on the proximity of our solar system to the galactic center. The closer we are, better we are.

One maha yuga is 12,000 celestial years, i.e., 4.32 million human years.
  • Krita yuga      4800 celestial years = 1,728,000 human years
  • Treta yuga      3600 celestial years = 1,296,000 human years
  • Dwapara yuga 2400 celestial years =    864,000 human years 
  • Kali yuga        1200 celestial years =    432,000 human years
We are currently believed to be in kali yuga, where the solar system is at its farthest (apogee) from the galactic center. Kali yuga is also known as the age of ignorance. In this yuga, life span of a healthy human is 100 yrs and their stature is 3.5 cubits (முழம்).

Dwapara yuga is known as the age of ambition. It preceded and succeeds the current kali yuga. Healthy humans are said to live 1000 yrs with a stature of 7 cubits. This might be an explanation for why the statues in pyramids are of 7 cubits scale. Lord krishna of hinduism is said to be from the dwapara yuga that preceded the current kali yuga.

Treta yuga is known as the age of virtue. With a healthy human lifespan of 10,000 years and 14 cubit statues and 3/4th of the people being enlightened, it sound beyond the imagination. Lord rama of hinduism is allegedly from the recent treta yuga.

Kirta yuga is know as age of truth, hence it is also referred as satya yuga. This is the when the solar system is closest (perigee) to the galactic center. As a result, Life span of 100,000 yrs and 21 cubit statures and every human is enlightened.

Every 71 maha yugas, the existing human race is wiped clear and a manu surfaces in the world in an epoch after universal destruction. The word 'man' is derived from this eastern word 'manu'. According to the puranasVaivasvata is the manu of current manvantara. His story is same as the story of Noah of christianity and gilgamesh of mesopotamia.

Each manvantara is preceded and followed by sandya, a time period of 4800 celestial years (1.728 million human years), where a partial creation (srishti) and destruction (laya) occurs.

Now we are entering the scale of whole cosmos. One full day of cosmos (bramha in hinduism) is called kalpa, known as aeon in the western part of the world. Currently we are said to be in sveta-varaha (white boar) kalpa. 

daytime of a kalpa = 14 manvantaras + 15 sandyas = 1000 maha yugas = 4.32 Billion human years
(followed by a night time of equal length)

As shown in the picture, one year of the cosmos (bramha) is 3.1104 trillion years.

Lifetime of the cosmos is allegedly 100 cosmic years, 311.04 trillion human years. A total shutdown of equal time span is expected after this and another big-bang and so on.

We (people of the 21th century AD), are in the middle of everything;

Allegedly, this kali yuga started in 3102 B.C. (18 Feb 00:00) - about 5100 yrs have passed, 427,000 more to go (as of 2000 AD).

Among the 14 manvantaras of this kalpa, we are in the 7th (Vaivasvata) and 7 more to go.

Among the 100 kalpas for a year of cosmos, we are at the 51st (sveta-varaha (white boar) kalpa).

311.04 trillion years translates to 1022 seconds. The same puranas denote that smallest indivisible life particle is called kalapa (not kalpa) and every living being is made of that, which closely resembles the latest discovery higgs-boson particle, also known as god particle with a life time of 10-22 seconds. isn't this more than just a co-incidence?

There is nothing to doubt or believe here. I'm just passing on what i've read, heard from independent sources and pondered over for years. Just reading this should give a good perspective of how vast the cosmos is and how insignificant are the objects of our vanity.

Related Theory: This may be very well be explaining the inner cycle of the yugas described above. As this whole is cyclic with just varying scale.

Some sources totally decline this celestial year concept. This celestial years was an adulterant concept introduced lately by aryans to exemplify it beyond human imagination and eventually make it consider more fictional. This theory emphasizes that its all actual human years, meaning, kali-yuga is 1296 human years, 108 years of pradosagama (dusk), 1080 years of actual kali-yuga and 108 years of sandhya (dawn). Going by that kali-yuga started in 3102 BC (krishna's demise), peaked at 1806 BC, started descending and ended on 510 BC (buddha's enlightenment). Dwapara yuga began then and it ends in 2082 and treta yuga begins. There will be significant global transformation during such conjunction of yugas. This video from a reliable source explains this point of view.