The Tibetan calendar and its variants
Calendar traditions
The Tibetan calendar is lunisolar. Its months follow a calculated lunar cycle, while leap months keep the numbered months related to the solar year. A year has twelve or thirteen months. Losar, Tibetan New Year, begins the first month, and the calendar also provides dates for Buddhist festivals, memorial observances and recurring practice days.
Tibetan calendar calculation developed from Indian astronomy associated with the Kalachakra, a Buddhist teaching tradition whose texts include a system of astronomical calculation. Tibetan scholars adapted these methods and developed several calculation traditions. The wider Tibetan almanac also draws on indigenous practices and Chinese-derived systems of elements and animal cycles.
Two principal calendar traditions are Phugpa, also called Phukluk, and Tsurphu, also called Tsurluk. Phugpa is widely used across Tibetan Buddhist communities. Tsurphu is associated especially with the Karma Kagyu tradition and takes its name from Tsurphu Monastery. These are calculation lineages, rather than two names for one universally identical calendar.
Their methods calculate average motions and apply corrections for variation in the Sun's and Moon's motion. Differences in starting values and leap-month rules can change month placement and daily labels. The traditional calculations use their own astronomical models, so their results need not coincide exactly with positions calculated using modern astronomy. Merely adjusting the time zone of a Chinese calendar does not produce a Tibetan one.
Skipped and repeated dates
A Tibetan month has thirty numbered lunar days, but these are not thirty equal periods of twenty-four hours. Each lunar day represents a 12-degree increase in the Moon's calculated longitude relative to the Sun. The time taken to complete one such interval varies.
The calendar assigns these numbers to ordinary days using a conventional dawn boundary. In the Phugpa rule, an ordinary day receives the number of the lunar day in progress at its beginning. A short lunar day can begin and end between two successive dawn boundaries, leaving no ordinary day with that number. A longer lunar day can span two boundaries, giving two successive ordinary days the same number.
An omitted date therefore means that a number is absent from the printed sequence. A repeated date means that the same number labels two different days. The weekdays continue normally through both cases. A repeated date can be distinguished by its first or second occurrence, its weekday and its corresponding civil date.
A leap month operates on a different scale. It adds a whole month to the year, giving two months the same number. In the Phugpa convention, the leap month comes before the ordinary month bearing that number. This order differs from the Chinese convention, in which a leap month follows the ordinary month whose number it repeats. The leap-month marker is therefore as important as the number itself.
Conversion records
A full Tibetan date includes its calendar tradition, year designation, month number, ordinary or leap-month status, lunar day and any repeated-date marker. Traditional year names combine an animal with an element in a sixty-year cycle. An animal name alone recurs every twelve years, while even a full cyclical name needs historical context to locate it in a particular century.
When converting a record, preserve the original date beside the civil equivalent. For a repeated date, distinguish the two occurrences. An omitted lunar date has no ordinary day carrying that label in the specified calendar, so it cannot be assigned a civil equivalent merely by counting forward from the start of the month.
Religious observance introduces a separate decision. Calendars may indicate when a practice normally associated with an omitted or repeated date is to be held. That observance rule belongs to the community or calendar concerned and does not change the underlying date sequence. A festival entry and a historical birth record can consequently require different handling even when they mention the same lunar day number.
The Atlas's Chinese converter does not calculate Tibetan dates. A Tibetan conversion requires a calendar or calculation for the relevant tradition, including its leap-month and repeated-date conventions.