In Jain tradition, Jain Dharma is अनादि (beginningless), so it does not have a single “start date.”

If you mean the oldest textual reference, the most commonly cited early reference is to Lord Ṛṣabha (Ādināth), the first Tirthankar, in the Rigveda. Within Jain scriptures, Lord Ṛṣabha is also described as the first Tirthankar and the original teacher of the path.

So, in simple words:

• Jainism itself: beginningless
• Oldest known textual reference: Ṛṣabha / Ādināth in the Rigveda