There are years.
Then there are groups of years, called yugas.
The shortest Yuga is a solar/lunar conjunction that happens every 5 years.
Then there is a 12 year yuga determined by the orbit of Jupiter.
And there are other types of small yugas, but there is another type of yuga that is not the generic sense of yuga (a junction of years), but is a specific entity, an unequal-fourth of a larger time-span called a kalpa.
This kind of yuga can be translated as “epoch” and the kalpa can be translated as an “aeon.”
These yugas are defined in multiples of 1,000 years. There are four of them:
- “x1” (kali) is the 1,000 year yuga
- “x2” (dvāpara) is the 2,000 year yuga
- “x3” (treta) is the 3,000 year yuga
- “x4” (kṛta) is the longest, best yuga, 4,000 years.
These four taken together make up a kalpa, but oddly a kalpa is not the sum of the years of the four yugas. It’s not 10,000 years, it is 12,000. The extra 2,000 years amas because of 10% transition periods on either border of each yuga.
Now, there are different ways to estimate the length of the yuga and kalpa – which depends on the type of “year” to be used. If you use a human year you see kali-yuga lasting 1,200 years (1,000 + two 100 year transition periods in and out of the yuga itself), dvāpara lasting 2,400, treta lasting 3,600 and kṛta (aka satya) lasting 4,800 years.
However there are other types of years, one is in ancestral time and the other is in heavenly time. Ancestral time is time from the point of view of the pitṛ (ancestral guardians). This is roughly 30 times longer than a human year, because the Moon’s waxing represents the ancestral day, and the Moon’s waning represents their night.
Heavenly time is time from the point of view of the deva (gods). This is roughly 360 times longer than a human year, because the Sun’s course north of the equator represents the gods’ daytime, and the Sun’s course south of the equator represents their night.
When we are talking about cosmic things, like durations of the cosmos itself, we should use heavenly time, because the gods occupy the outer cosmos. Thus, usually, the four-yugas are actually defined as 360 times longer what you might expect. Thus in human years:
- kali = 432 thousand years
- dvāpara = 864 thousand years
- treta = 1.296 million years
- kṛta = 1.728 million years
And the sum total of the four of them, equalling a kalpa is: 4.32 million years.
Next, longer than a kalpa is a mānvantara. There are just about 14 kalpa in a mānvantara. So its roughly 60.4 million years. This timescale has to do with the amount of time that mankind is in a certain state or generation from a certain forefather and foremother.
Larger than a mānvantara is a “day of the Creator (Brahmā).” This day consists of 1,000 kalpas! So it lasts about 4.32 billion years. Following every Day of the Creator is a Night of the Creator. During the day, creation appears and expands. At night it contracts and disappears. So this is like a whole life cycle of a solar system. Its duration taken together is 8.64 billion years.
There are 360 days/nights in a Creator’s Year, and he exists for 100 such years. So a creation itself, a “universe,” exists for 311.04 trillion human years.
Universes also go through existence and non-existence cycles with the whole thing totaling 622.08 billion years. The existence of a universe corresponds not even to the duration of an inhalation on the spiritual timescale, which is infinite.
Another trippie, cool thing is that the basic pattern of events repeats itself. Just like Winter, Spring, Summer, Fall always follows a predictable pattern. So the same basic types of events always unfold in each yuga, each kalpa, each manvantara, each Day of Brahmā, and even in each universal cycle.
This description I’ve given is standard and found consistently throughout the 18 major Puāṇa.