Here we briefly describe the Tibetan calendar. The Tibetan calendar is a unique system which is lunar-based. Tibetan calendar making is based on the Sri Kalachakra Tantra (Wheel of Time Tantra), which was translated into Tibetan from Sanskrit in 1027 A.D. According to Tibetan Calendar, a year has only 360 days and a complicated system of using skip-days (tsi chad-pa) and extra days (tsi lhag-pa). The new moon always starts the month, and the full moon falls on the fifteenth day. As the lunar phase defines a month, every month is 30 days and a year has twelve months. The lunar day is about one minute shorter than a solar day, which is 24 hours. To make the calendar correspond with solar days, it occasionally adds a 13th month.
The official Tibetan calendar is lunar, but in Tibetan calendar making different systems are used, which harmonize solar and lunar factors. Each year is ruled by one of the five elements and one of twelve animal signs as in Chinese calendars, but they start the year on different dates and the months have different lengths. So it is very important not to mix Tibetan and Chinese systems together. The main purposes of the Tibetan calendars: (1) providing a working seasonal schedule for farmers, shepherds, etc. (2) dating the enlightening date of Buddha. (3) computing the moon and sun eclipses. Therefore, it was important to predict the timing of them for a believer to meditate or study.