Bhagavata Purana clearly states that Lord Rama became king during Treta yuga (Bhag. 9.10.51). We have been in Kali-yuga for 5000 years. Before this was Dvapara-yuga which lasts 864,000 years. Before that was Treta-yuga, which lasts over 1,200,000 years. Thus, according to this, the existence of Lord Rama had to have been many thousands of years ago. And if Lord Rama appeared in one of the previous Treta-yugas, it would certainly indicate that Lord Rama appeared several million years ago. And this is exactly what is corroborated in the Vayu Purana.
In the Vayu Purana (70.47-48) [published by Motilal Banarsidass] there is a description of the length of Ravana’s life. It explains that when Ravana’s merit of penance began to decline, he met Lord Rama, the son of Dasarath, in a battle wherein Ravana and his followers were killed in the 24th Treta-yuga. The Roman transliteration of the verse is:
tretayuge chaturvinshe ravanastapasah kshayat
ramam dasharathim prapya saganah kshayamiyavan
The same Ramayana that gives the planetary descriptions (on which the above calculation is based) also tells that Lord Rama came in Treta-yuga. The Matsya Purana (47/240,243-246) is another source that also gives more detail of various avataras and says Bhagawan Rama appeared at the end of the 24th Treta-yuga.
There are 1000 Treta-yugas in one day of Brahma, and it is calculated that we are presently in the 28th cycle of the four yugas (called divya-yugas, which is a cycle of the four yugas, Satya-yuga, Treta-yuga, Dvapara-yuga, and then Kali-yuga) of Vaivasvata Manu, who is the seventh Manu in the series of 14 Manu rulers who exist in one kalpa or day of Brahma. Each Manu is considered to live for 71 such divya-yuga cycles. So, without getting too complicated about things, from the 24th Treta-yuga to the present age of this 28th cycle of Kali-yuga, there is obviously a difference of millions of years when Lord Rama manifested here on earth. This gives the period of Lord Rama approximately 18 million years ago. Furthermore, the planetary positions mentioned in the Ramayana would also have occurred multiple times in history prior to the calculated date. Of course, few people may believe this unless they are already familiar with the vast lengths of time that the Vedic literature deals with.
Laghu Bhagavatamrta Text 78 says :
kausalyayam dasarathan
nava-durva dala-dyutih
tretayam avirabhavac
caturvimse catur-yuge
bharatena sumitraya
nandanabhyam ca samyutah
kausalyayam - in the womb of Kausalya-devi; dasarathat - from Dasaratha Maharaja; nava - fresh; durva-dala - durva grass; dyutih - with the splendor; tretayam - during the Treta-yuga; avirabhavat - appeared; caturvimse - during the 24th; catuh-yuge - cycle of four yugas; bharatena - by Bharata; sumitrayah - of Sumitra; nandanabhyam - by the two sons (Laksmana and Satrughna); ca - also; samyutah - accompanied.
"Splendid as a new blade of durva grass, and accompanied by Sumitra's two sons and by Bharata, Lord Ramachandra appeared in the Treta-yuga of the 24th catur-yuga as the son of Kausalya and Dasaratha."
It is clearly mentioned that caturvimse catur yuge i.e. during the 24th caturyuga.
Calculations: 1 Catur-yuga = 4,320,000 years or 43 lakh 20 thousand years.
Catur-yugas passed = 24th part (roughly half) of Treta & full Kali, 25th, 26th, 27th, 28th till part of Kali-yuga.
Years passed approx. = 432,000 + 432,000 + 4,320,000 + 4,320,000 + 4,320,000 + (4,320,000-427,000 left)
= 17,717,000 (+ or - 432,000 years)
(+ or - 432,000 years is required because we don't exactly know in which period of Treta Yuga of the 24th Chatur-yuga Lord Rama appeared).
Nonetheless, maybe there is further reason why we should accept that Lord Rama appeared millions of years ago. In the Valmiki Ramayana, Sundara-Kanda (or Book 5), Chapter 4, verse 27, [Gita Press, Gorakhpur, India] it explains that when Hanuman first approached Ravana’s palace, he saw the doorways surrounded by horses and chariots, palanquins and aerial cars, beautiful horses and elephants, nay, with four-tusked elephants decked with jewels resembling masses of white clouds.
Elsewhere in the Valmiki Ramayana, Sundara-Kanda (or Book 5), Chapter 27, verses12, an ogress named Trijata has a dream of Lord Rama, which she describes to the other demoniac ogresses upon awakening. In that dream she sees Rama, scion of Raghu, united again with Sita. Sri Rama was mounted on a huge elephant, closely resembling a hill, with four tusks.
The question is how could there be a mention of the elephants with four tusks unless Valmiki and the people of his era were familiar with such creatures? A quick search on the Encarta Encyclopedia will let us know that these four-tusked elephants were known as Mastodontoidea, which are said to have evolved around 38 million years ago and became extinct about 15 million years ago when the shaggy and two tusked Mastodons increased in population. Now there’s something to think about, eh?
taken from here