An internal evidence from Bṛhadāraṇya Upaniśad on timing of Yājñavalkya
The epic Mahābhārata states Maitreyī is a young beauty who is an Advaita scholar but never marries — states John Muir, (Metrical Translations from Sanskrit Writers, p. 251, at Google Books, page 251–253)
So, we have the memory of Yājñavalkya — Maitreyi in Mahābhārata. Vaiśampāyana is the Ṛshi for Yajur Veda and Yājñavalkya was one of his foremost disciples. Vaiśampāyana was himself a pupil of Vyāsa.
In one of my last talk I had shared that the Gārgī and Maitreyī were not Ṛgvedic Ṛṣikās. They are not the mantradraṣṭās of Ṛg Veda. They are the adhikārīs of Vedas. We also know that Yājñavalkya started the school of Shukla Yajurveda, as he had to give away all the Vedas and he felt empty handed in Knowledge.
There are prose in Bṛhadāraṇya Upaniśad stating Saṃvatsara and naming of Aditi Nakshatra
आपो वा अर्कः; तद्यदपां शर आसीत्तत्समहन्यत । सा पृथिव्यभवत्, तस्यामश्राम्यत्; तस्य श्रान्तस्य तप्तस्य तेजो रसो निरवर्तताग्निः ॥ २ ॥
2. Water is Arka. What was there (like) froth on the water was solidified and became this earth. When that was produced, he was tired. While he was (thus) tired and distressed, his essence, or lustre, came forth. This was Fire.
स त्रेधात्मानं व्यकुरुत, आदित्यं तृतीयम्, वायुं तृतीयम्, स एष प्राणस्त्रेधा विहितः । तस्य प्राची दिक् शिरः, असौ चासौ चेर्मौ । अथास्य प्रतीची दिक् पुचम्, असौ चासौ च सक्थ्यौ, दक्षिणा चोदीची च पार्श्वे, द्यौः पृष्ठम्, अन्तरिक्षमुदरम्; इयमुरः, स एषोऽप्सु प्रतिष्ठितः; यत्र क्व चैति तदेव प्रतितिष्ठत्येवं विद्वान् ॥ ३ ॥
3. He (Virāj) differentiated himself in three ways, making the sun the third form, and air the third form. So this Prāṇa (Virāj) is divided in three ways. His head is the east, and his arms that (north-east) and that (south-east). And his hind part is the west, his hip-bones that (north-west) and that (south-west), his sides the south and north, his back heaven, his belly the sky, and his breast, this earth. He rests on water. He who knows (it) thus gets a resting place wherever he goes.
सोऽकामयत, द्वितीयो म आत्मा जायेतेति; स मनसा वाचं मिथुनं समभवदशनाया मृत्युः; तद्यद्रेत आसीत्स संवत्सरोऽभवत् । न ह पुरा ततः संवत्सर आस; तमेतावन्तं कालमबिभः, यावान्संवत्सरः; तमेतावतः कालस्य परस्तादसृजत । तं जातमभिव्याददात्; स भाणकरोत्, सैव वागभवत् ॥ ४ ॥
4. He desired, ‘Let me have a second form (body).’ He, Death or Hunger, brought about the union of speech (the Vedas) with the mind. What was the seed there became the Year (Virāj). Before him there had been no year. He (Death) reared him for as long as a year, and after this period projected him. When he was born, (Death) opened his mouth (to swallow him). He (the babe) cried ‘Bhāṇ!’ That became speech.
Apparently there is a connection between East and Year beginning, which can be read in verse 3 and 4. Astronomically it is understood that the true East can be found only on the equinox days. In the next prose-verse Aditi is remembered as the heaven. The prose-verse is very cryptic, yet has the sense of Prajāpati’s connection with Aditi and Saṃvatsara. Lets read the verse below.
स अइक्षत, यदि वा इममभिमंस्ये, कनीयोऽन्नं करिष्य इति; स तया वाचा तेनात्मनेदं सर्वमसृजत यदिदं किंच — ऋचो यजूंषि सामानि छन्दांसि यज्ञान् प्रजाः पशून् । स यद्यदेवासृजत तत्तदत्तुमध्रियत; सर्वं वा अत्तीति तददितेरदितित्वम्; सर्वस्यात्ता भवति, सर्वमस्यान्नम् भवति, य एवमेतददितेरदितित्वं वेद ॥ ५ ॥
He thought, ‘If I kill him, I shall be making very little food.’ Through that speech and that mind he projected all this, whatever there is — the Vedas Ṛc, Yajus and Sāman, the metres, the sacrifices, men and animals. Whatever he projected, he resolved to eat. Because he eats everything, therefore Aditi (Death) is so called. He who knows how Aditi came to have this name of Aditi, becomes the eater of all this, and everything becomes his food.
By now we understand that the Aditi is equated with the food — Anna.
द्वया ह प्राजापत्याः, देवाश्चासुराश्च… (similar verses comes in shatapatha also)
Two’ here means two classes. The particle ‘ha’ is an expletive referring to a past incident. It is here used to recall what happened in the past life of the present Prajāpati. Of Prajāpati’s sons, in his past incarnation. Who are they? The gods and the Asuras,…
The above mentioned information very aptly mentions the parts of a year, where half of it belongs to Devas and half of it as Asuras. Likewise, Prashnopaniśad says that in each Saṃvatsara, i.e., year, there are two ‘Ayanas’ mentioned — and … ‘ṭatah annam vai retas’ and ‘tasmāt prajāyante imaaH prajāḥ’
अन्नं वै प्रजापतिस्ततो ह वै तद्रेतस्तस्मादिमाः प्रजाः प्रजायन्त इति ॥
अन्नं वै प्रजापतिः ततः तत् रेत जायते तस्मात् इमाः प्रजाः प्रजायन्ते ॥
“Food is (verily) the Eternal Father; for of this came the seed and of the seed is the world of creatures born .— Prashnopaniśad, first question and 14th verse
प्रजापतिः वै संवत्सरः — Prashnopaniśad
In sanskrit, especially in Brahmanas ‘vai’ means verily. Here the chronology of the year has been explained w.r.t the seed and its springing into prajā thereafter, that is his offspring (prajā in sanskrit means children). Spring is the season to sprout.
Other information through the lens of Nakshatra we know that Aditi is the deity of Punarvasu and if Aditi is rising in the east and casting herself with Prajapati then it can only mean that the vernal equinox is happening in Punarvasu. This takes us to the time period of 5300 BCE~6260 BCE. We know from above that Yājñavalkya and Maitreyi in Mahābhārata are contemporary of Vyāsa.
Shri Nilesh Oak, with his works, have proved that Mahābhārata war happened in 5561 BCE. The evidence from Bṛhadāraṇya Upaniśad also endorses the time period of the vernal in Punarvasu very-well.