The Kālacakra Tantra is more properly called the Kālacakra Laghutantra, as it is said to be an abridged form of an original text – the Kālacakra Mūlatantra.
It is said by the Tibetan historian Tāranātha, that the Mūlatantra was taught by the Buddha on the full moon of the month Caitra in the year following his enlightenment, at the great stupa of Dhānyakaṭaka (dpal ldan 'bras spungs kyi mchod rten) in India. This teaching had been requested by the king Sucandra from Sambhala (often written "Shambhala". In original Sanskrit texts the form is usually equivalent to "Sambhala", but in Tibetan texts it is written equivalent to "Shambhala".)
Mañjuśrī Yaśas, said to be the author of the Kālacakra Laghutantra
Sucandra returned to Sambhala and wrote the Tantras in textual form there. He composed the explanatory Tantra in 60,000 lines as a commentary on the original Mūlatantra of 12,000. A later king of Sambhala, Yaśas, wrote the abridged form of the Tantra, the Kālacakra Laghutantra. This is about one quarter of the length of the original Mūlatantra. This text survives today, and is generally known simply as the Kālacakra Tantra.
The next king was Puṇḍarīka, and he composed a commentary on the Laghutantra known as the Vimalaprabhā. This also survives to this day, and both these texts are available in the original Sanskrit and Tibetan translation. However, the original Mūlatantra, if it ever existed, has been lost, although significant sections remain in quotations in the Vimalaprabhā and some other texts. The existence of these quotations does not of course prove that the Mūlatantra ever existed as a complete text.