It's difficult to date Hinduism because as per the scriptures, Hinduism (the Sanātana-dharma) has no beginning, no end.
Hinduism, aka Sanātana-dharma (सनातनधर्म), literally, by definition itself, is Eternal. It always was, is and always will be.
This idea is brought out most succinctly by God Krishna himself in the Srimad Bhagvad Geeta.
न त्वेवाहं जातु नासं न त्वं नेमे जनाधिपाः ।
न चैव न भविष्यामः
सर्वे वयमतः परम् ॥ 2.12 ॥
- Nor at any time indeed was I not, nor thou, nor these rulers of men, nor verily shall we ever cease to be hereafter.
Thus, space-time having cyclic - repetitive nature in Hinduism, what we have is eternal nature of the Universe, and thus no beginning.
This idea is dwelled upon further in the Srimad Bhagvad Geeta 8.18 to 8.20.
And further, BG 9.7 & BG 9.8 also.
सर्वभूतानि कौन्तेय प्रकृतिं यान्ति मामिकाम् ।
कल्पादौ विसृजाम्यहम् ॥ 9.7 ॥
- All beings, O Arjuna, go into My Nature at the end of a Kalpa; I send them forth again at the beginning of (the next) Kalpa.
प्रकृतिं स्वामवष्टभ्य विसृजामि पुनः पुनः ।
कृत्स्नमवशं प्रकृतेर्वशात् ॥ 9.8 ॥
- Animating My Nature, I again and again send forth all this multitude of beings, helpless by the force of the Nature.
English Translation By Swami Sivananda
Thus, Hinduism being eternal, there cannot be any dating of Hinduism, at least not from any scriptural or traditional-orthodox perspectives.