Chinese astrological and astronomical calculations appeared in Tibet before Indian ones. This happened in the middle of the seventh century, during the reign of Emperor Songtsen Gampo, founder of the great Tibetan Empire. His wives were a Chinese princess and a Nepalese princess; the former brought various Chinese astrological, astronomical and medical texts with her to Tibet. A couple of years later, the Tibetan court was already designating years by the names of twelve animals, but the 60-year cycle was not yet in use. For the next two centuries this system remained practically the only one in Tibet.
After a period of general cultural decline in the ninth century, a new wave of Chinese astrological and astronomical influence came to Tibet from the Khotanese region of East Turkestan in the tenth century. The Tibetan master Dharmakara combined the innovations with the information that he and others remembered from the previous period of dissemination of these teachings, which by then had been noticeably distorted. He compiled a new complete system of calculating the elements: it now included calculations related to death, marriage, obstacles, horoscope calculations, and geomancy. By the eleventh century, Tibetans were mostly using the 60-year cycle of animals and elements.
The modern Tibetan calendar also has a “royal” year number. This is the number of years since the ascension to the throne in 127 AD of the first Tibetan king Nyatri Tsengpo.
The Indian part of astrological and astronomical calculations appeared in Tibet along with the “Kalachakra Tantra”. Various translators and masters translated the basic Kalachakra texts from Sanskrit and transmitted them to Tibet several times between the eleventh and thirteenth centuries. These texts became important in the early Sakya and Kagyu traditions, and many commentaries were later written on them. In addition, the works of Chinese and Indian masters were combined and reworked, resulting in a distinctive Tibetan astrological and astronomical tradition.
As in Hindu systems, the Kalachakra uses the 60-year cycle of Jupiter to count the years. This cycle is called rabjung, or “important” cycle, after the first of the sixty years.
The first year of the first “important” 60-year cycle of the Tibetan calendar is considered the official year of the appearance of the Kalachakra in Tibet. This is the famous, fire-space-ocean number of years predicted in the Kalachakra literature after the beginning of the Muslim period in 624 CE, although the Muslim period actually began in 622.
Both the Kalachakra and Hindu systems give names to numbers, using a list common in Panindya literature, and give them in a certain order: ones, tens, hundreds, and so on. There are three fires and four oceans, and space is as empty as zero. Thus, “fire-space-ocean” is 403 years after the 624th, which is the year 1027.
When the 60-year “important” cycle of Kalachakra was compared to the Chinese 60-year cycle of elements and animals, the year 1027 did not coincide with the beginning of the Chinese cycle. The Chinese cycle always begins with the masculine wooden year of the rat, and 1027 fell in the fourth year of the cycle, the feminine fiery year of the hare. Therefore, the Tibetan 60-year cycle begins with the female fire year of the hare and the list of the twelve animals begins with the hare, not the rat. Because of the three year difference, the current Tibetan 17th cycle began in 1987, while the current Chinese 27th cycle began in 1984.
Although the first “important” 60-year cycle began in 1027, the Kalachakra calendar was not legitimized in Tibet until the second half of the 13th century. Nevertheless, many people still labeled the year using the name of the element and animal – this is still popular today – rather than its name in the “important” cycle. In doing so, the mathematical calculations for the calendar were taken from the Kalachakra system.
One of the outstanding early masters of the Sakya school and authors of astrological studies was Chogyel Phagpa, who lived in the second half of the 13th century. He was the tutor of the Mongol ruler of China, Khubilai Khan (Khublai Khan), and the spiritual master who, together with his uncle Sakya Pandita, was entrusted with bringing Tibetan Buddhism to Mongolia. As a renowned master of the Kalachakra teachings, Chogyel Phagpa undoubtedly also brought the complete Tibetan astrological and astronomical system to Mongolia. Moreover, the Kalachakra calendar was officially adopted in Tibet, probably because the Mongol Khans, who, beginning with Khubilai, were emperors of the Yuan dynasty in China, appointed first Chogyel Phagpa’s uncle and then himself as secular ruler of Tibet.
At the beginning of the thirteenth century, Genghis Khan, the grandfather of Khubilai Khan, had already adopted from the Uighurs the calculation of years by means of the twelve-year animal cycle and legitimized it in his empire. According to one of the descriptions, on the occasion of the conquest of the Tangut kingdom in 1207 – today it is eastern Gansu and Inner Mongolia – Genghis Khan introduced “Mongolian months”, which corresponded to and replaced the Chinese ones.
When Genghis Khan’s successors introduced the Tibetan calendar into the Mongol Empire in the middle of the same century, they changed the Mongolian months to match not the Chinese but the Kalachakra months, which are very different. However, the first Mongolian month remained the first month of the year, which is in accordance with Chinese tradition – even though it starts two months earlier than the first Kalachakra month. This was adopted by Tibet, meaning that throughout the Mongol Empire the new year began at roughly the same time. However, the beginning of the Chinese and Tibetan new year did not always coincide because each of these calendar systems has its own mathematical formulas for adding leap months and determining the beginning and duration of each month. In Tibet, the Mongolian months were called “Tibetan months,” and even today the two names are interchangeable.