Tibetan Calendar

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.

The calendar sets the ritual year for Tibetan Buddhism far beyond the new year that opens it. Saga Dawa, the fourth lunar month, is held to be the month of the Buddha's birth, enlightenment and parinirvana together, and merit generated during it is traditionally reckoned to be multiplied; Lhabab Duchen marks the Buddha's descent from the Tushita heaven, and a run of other duchens, "great occasions," are fixed to specific lunar dates rather than to the solar calendar most of the world now keeps. Because the reckoning is lunisolar and computed rather than merely counted, these dates shift against the Gregorian calendar from year to year, and a doubled or omitted day in a given month can move a festival by a full day within Tibetan communities that otherwise share it.

The calendar is not a historical relic. It is kept daily by Tibetans in the Tibet Autonomous Region, by the diaspora centered on Dharamsala, and by Buddhist communities in Bhutan, Nepal, Sikkim, Ladakh and Mongolia, where Losar is still the year's principal family and religious festival, marked with home rituals, monastic ceremonies and the reading of astrological forecasts drawn from the same almanac tradition that fixes the calendar's dates. Tibetan almanacs, tsel, are published annually and consulted for auspicious days for travel, marriage and ordination alike, so the calendar remains as much a living divinatory tool as a civil one.