Porhalaan
The Porhalaan or Batak calendar is the traditional calendar of the Batak living on the Indonesian island of Sumatra . It is an imputed solar calendar with 12 months of 30 days each and an occasional leap month. It is also known as the “magic calendar” because it is used to distinguish between days that are not very lucky and those that are more favorable. It is not used for calculating time itself. The year begins on the new moon in May, when the constellation Orion disappears in the west and the constellation Scorpio ascends in the east.
The management of the calendar is traditionally entrusted to a specific person, the so-called datu porrusukrusuk (short: datu ), who is therefore also responsible for the selection of favorable days for celebrations, the start of a company and the like. In order to minimize the risk of accidentally selecting an unfavorable day due to errors in the calendar management, days are often selected that promise happiness in two months, the presumably current month and the following one. Therefore, there is often an extra 13th month in the calendar that serves this purpose. If this is not available, the first month is simply used again for protection. It is possible that the 13th month was also used to compensate for the difference to the solar year, but this has not been proven.
The Porhalaan is often written as a table of square fields of 30 columns (days) of 12 or 13 lines (months) each on a cylindrical piece of bamboo.
The datu uses a piece of buffalo rib to keep the calendar in which two rows of 12 holes each for the months and two rows of 30 holes each for the days have been worked. Individual days and months are counted by threading a thread.