When was the Mahabharata written?

The Mahabharata was not written once. It grew, and the text says as much: a core called Jaya, an expanded Bharata of twenty-four thousand verses, and finally the hundred-thousand-verse Mahabharata. Any date therefore applies to a layer, not to the book.

The scholarly consensus, insofar as one exists, is that the heroic core took shape in oral form in the centuries before the common era, that the text was substantially assembled between about 400 BCE and 400 CE, and that the didactic books, the Shanti and Anushasana Parvas above all, together with the framing narratives and much of the Krishna theology, belong to the later part of that window. Some scholars argue for earlier oral origins reaching back several centuries further; others emphasise that the surviving text is a product of the Gupta period.

The evidence is indirect. The epic is mentioned by name in the Ashvalayana Grihyasutra, a work of uncertain but early date. The grammarian Panini, generally placed in the fourth century BCE or earlier, knows the names Yudhishthira and Arjuna. An inscription from Cambodia of the sixth century CE refers to recitation of the Mahabharata, which shows the text was fixed and travelling by then. None of these fix a moment of authorship.

Traditional chronology works differently. It places the war and the composition at the junction of the Dvapara and Kali yugas, a point calculated in later astronomical literature as 3102 BCE. This is a religious and calendrical reckoning rather than a historical one derived from manuscripts, and it should not be blended with the scholarly dates as if the two were measuring the same thing.

The honest summary: the Mahabharata was composed over centuries, the process is better attested than any date, and the disagreement about its beginning is genuine.