How today's Nepali date is decided
Bikram Sambat is a solar calendar. Each month starts at sankranti, when the sun enters a new rashi. This sets how many days the month has, from 29 to 32. The official yearly patro published in Nepal fixes these month lengths, and Katigate uses the same tables instead of a formula. This is why the AD date for a given BS date, such as Asoj 1, can move by a day from one year to the next.