Tibetan month (Tib. ཧོར་ཟླ་, horda, Wyl. hor zla) is the name we use in English for the months of the Tibetan calendrical year, as opposed to the months of our Gregorian calendar. Its name in Tibetan hor zla means "Mongol month", as it was borrowed from the Mongolian way of calculating months.
These are not exactly lunar months as their calculation is based on both the position of the moon and that of the sun. Hence, the Tibetan calendar is known as a lunisolar calendar. In this calendar, the new moon always begins the month and the full moon falls on the 15th day of the month.
However, since the lunar year has only 354 days, a very complicated astrological calculation is used to make each month have 30 days and each year 360 days in order to reconcile the differences between the solar and lunar cycles. This is achieved by adding an extra day to certain months and omitting certain days in certain months. Sometimes, dates appear twice in a row. When that happens, it is compensated for with another date elsewhere in the calendar that had been omitted. Every three years, a 13th month is added anywhere amongst the twelve months based on astrological calculations as to where it is deemed most auspicious for that particular year. This is done so that the calendar corresponds with the seasons.
From the 11th century to the second half of the 13th century, Tibetan months were named according to lunar mansions. However, from the second half of the 13th century, the Tibetan ruler Drogon Chopal Phagpa introduced the system of counting the months by numbers, for example, the 1st month became Dawa Dangpo; the 2nd month became Dawa Nyipa, and so on.
- Jampa Yangchen, "Losar, Guthuk and More", Tibet Foundation Newsletter No. 74, Spring 2019