Short answer: In Jain Dharma, Jainism did not “start” at any point in time. It is anādi–ananta dharma—beginningless and eternal.

A little context:

• Time is cyclical. In every time-cycle, Tirthankars appear to re‑establish the path. In the current descending cycle (avasarpiṇī), the first Tirthankar is Bhagwān Ṛṣabhdeva (Ādināth).
• Historically attested in our era, Bhagwān Mahāvīr (24th Tirthankar) revived and organized the fourfold Saṅgha in the 6th century BCE. Traditional Jain dating places His nirvāṇa on Kārtik Amāvasya in 527 BCE. Scholars may differ on exact BCE years, but this does not affect the core Jain view that the Dharma itself is eternal.
• Both Śvetāmbara and Digambara traditions agree that Jain Dharma is beginningless and that Ṛṣabhdeva is the first Tirthankar of this cycle. Minor variations may exist in calculated historical years, but not in this principle.