Calendrical cycle or Sexagenary cycle, (Tib. rabjung)--the sixty year cycle followed in the Tibetan calendar, which is a complete cycle according to Tibetan astrology.
Accordingly in the Tibetan system, years are identified by one of the 60 names formed with an element (earth, fire, water, wood, and iron) and an animal (hare, dragon, snake, horse, sheep, monkey, bird, dog, pig, mouse, ox, and tiger) identified, and the number of the sexagenary cycle. The first sexagenary cycle was in 1027 A.D. when this Tibetan dating system was instituted by Lotsawa Dawa Özer. We are now in the 17th cycle. So for example, the year 1959 was 'approximately' the Earth Pig year of the 16th cycle. 'Approximately' because the Tibetan New Year is usually somewhere between end of January and beginning of March. So the first days of January 1959 where still in the Earth Dog year of the 16th cycle (the 32nd year of a sexagenary cycle) while January 1960 was part of the Tibetan Earth Pig year (the 33rd year).