The traditional Tibetan calendar is a lunar one based upon the monthly cycles of the moon as it appears and disappears across the sky. The first day of the month then is when a new moon appears, the 15th of each month is the appearance of the full moon, and the 30th and final day of each month is when the moon has disappeared from the sky. Thus a calendar year has 12 thirty day months. The most Tibetan holiday was traditionally the lunar new year, called “Losar” (lo gsar), literally “year-new”.

The broader calendar is then organized into 60 year cycles referred to as “rabjung” (rab byung), which roughly corresponding to the international notion of a “century” (100 years). Each of them internally consists of 5 twelve year subcycles, which roughly correspond to the international notion of a decade.  Each year within a 60 year cycle has a unique identity based upon a systematic combination of animal signs, elemental energies, and gender.  A given twelve year cycle thus may thus proceed through the hare (yos), dragong (‘brug), snake (sbrul), horse (rta), sheep (lug), monkey (sprel), bird (bya), dog (khyi), pig (phag), mouse (byi), ox (glang), and tiger (stag). These are combined with one of the five elements – fire (me), earth (sa), iron (lcags), water (chu), and wood (shing). Each year then also changes gender – male (pho) or female (mo). Typically, the same element will apply for two years in a row, while the animal will change each year. The full combination for a given year is given as element-gender-animal.  The unique combinations of element and animal thus add up to 60 permutations, and then it all restarts again.  Thus to calculate a specific date, you must know which 60 year cycle is in question – the first, second, third, and so forth.

The problem is that there is more than one calendar system in Tibet, and each starts their enumeration of 60 year cycles on a different date, as well as differing in other details. Thus to assess a specific date, you have to know which calendar system has been used to calculate it.