The Tibetan calendar is lunisolar, built on the astronomical system the Kalachakra Tantra brought into Tibet from Indian sources beginning in the eleventh century and reworked over the following centuries into a distinctly Tibetan science. Months follow the moon, twelve to a year, and each day of a month corresponds to a tithi, one thirtieth of a lunation measured by the angle between sun and moon rather than by the clock. Because a tithi's true length varies, some run longer than a solar day and some shorter, so a Tibetan month occasionally repeats a day number or skips one outright: a doubled fifteenth can sit beside an omitted sixteenth in the same month, a mechanism with no equivalent in the Gregorian reckoning. To keep the lunar year in step with the solar one, an intercalary month is inserted roughly every three years, chosen by the same astronomical calculation rather than a fixed rule.
Several monastic traditions compute these tables independently. The Phugpa system, descended from the fifteenth century astronomer Phagpa Lotsawa's reworking of the Kalachakra tables, is the reckoning most Tibetan Buddhist communities follow today, though the Tsurphu tradition of the Karma Kagyu school sometimes reaches a different intercalation and can set its new year, or a festival, a day or a month apart from the calendar kept elsewhere. Losar, the new year, opens the count each spring.