When did John Marshall, Director-General of the ASI, announce the discovery of a new civilisation in the Indus valley to the world?

1924

Archaeologists like Daya Ram Sahni discovered seals at Harappa in the early 20th century, in layers much older than the Early Historic period, leading to the realization of their significance.
Rakhal Das Banerji found similar seals at Mohenjodaro, suggesting that these sites were part of a single archaeological culture.
In 1924, John Marshall, Director-General of the ASI, announced the discovery of a new civilization in the Indus Valley to the world, based on the findings of the seals. This revealed a civilization contemporaneous with Mesopotamia.
Change in Indian archaeology: John Marshall's tenure marked a significant change in Indian archaeology. He was the first professional archaeologist to work in India, bringing his experience from Greece and Crete.