Kamal Prakash Malla (KP Malla) was an academician of Newa studies. The snips are taken from an email communication where KP Malla comments about a publication by the New Year Celebrations Committee, i.e. Nhoo Dan Samaroha Samiti. Here are some keynotes about Nepal Sambat and the tithi / lunar cycle from the commentaries.
All that we know for sure is that the epoch era was founded at the time of Raghavadeva, on Tuesday, October 20, 879.
The lunar calender was invented by Babylonians in ca. 4000 BC. It was brought to the Indus Valley by the nomadic Indo- Aryans in about 1350, BC.
Calculation: Unlike the solar calender ( in which a civil day is calculated from mid-night to mid-night or from a sun-rise to another sun-rise), the lunar calendar is based on the phases of the Moon or periods of “lunation” which in Hindu calendar is called “the tithi”. Its duration is totally inconsistent and unpredictable so that a tithi may end and another begin at any moment, any second, in a solar day.
Unpredictability: This gives rise to the problem of deciding WHEN does a tithi end. In order to know the ending moment of a tithi (which is the most crucial for calculation), one has to carry a pocket almanac, as it were, since it is simply unpredictable!
Inconsistency: Secondly, a lunar tithi may span longer or shorter than a solar/civil day. Thirdly, a lunar tithi may be doubled or lost. The same civil day may contain more than one lunar tithi.
Additional month: Fourthly, in order to adjust the two reckonings, every two-and-a-half year, there has to be an intercalary month, or additional lunar month, because the lunar year is shorter by 11.25 days than the solar year. This also gives rise to the problem of a “lost lunar month”. As lunar calendar is so fraught with practical problems it is not adopted as official one by any modern government in the world. However, it is still the RITUAL calendar in India, China and Islamic countries all over the world.