"There are many versions of how the Kalachakra Tantra came to India. According to Ra and Dro traditions, the tantra is practiced in the 7th century but secretly and later it became popular during the time of Padita Cilupa of Nalanda in the 9th century," says Venerable Tenzin Gawa, a research scholar on Kalachakra from Namgyal monastery.
The root text of Kalachakara tantra and its commentary compiled by the Shambhala King Suchandra was extinct but the commentary compiled by Kalki Manjushri and its commentary "Vimalaprabha" by Kalki Pundarika are extent.
The Kalachakra Tantra was first translated into Tibetan in 1027 and the Rabjung system of Tibetan calendar was also introduced in the same year.
Full explanation below: