In the 7th century, Princess Wen-Cheng brought Tang's calendar to Tibet, later Princess Jin-Cheng did the same thing. Apparently, they did not bring with them the theoretical Astronomy which the calendars were based on. Those Tibetan students who were sent to Tang to study Arithmetics did not learn the computations to make calendars. After the down fall of the Tibetan Dynasty, Tibet became chaotic, and the transmissions of the Han calendars stopped, Tibetan were left without a scientific calendar until 11th century.

Around 9th century, Islam extended to India, many Indian Buddhists escaped to Tibet with `aru-yoga' of the Esoteric (Tantric) Buddhism. Its part of `Kala (time) Wheel' was a part of the Indian Astronomy, and not up to the early Tang's achievement. Tibetan fused the Indian Astronomy, the Tibetan creativities and the Han calendars (rgya-rtsis) to form their own calendars, later they adopted more from the Han calendars to form the present Tibetan calendars.

The main purposes of the Tibetan calendars were (1) providing a working seasonal schedule for farmers, shepherds, etc. (2) dating the enlightening date of Buddha, for Buddha propheted that Buddhism would exist for 5,000 years (different schools might have different figures). If we knew the precise date of the enlightening of Buddha, then the end of the world of Buddhism would be open to us. The important clue was that there was a complete moon eclipse on the enlightening day of Buddha. (3) computing the moon and sun eclipses. The complete moon eclipse on the date of the enlightening of Buddha indicated the importance of moon or sun eclipses which should increase one's intelligence 1 million times. Therefore, it was important to predict the timing of them for a believer to meditate or study.