Bikram Sambat (abbreviated as B.S. or वि.सं.) is the official historical and national calendar of Nepal. It is approximately 56 years and 8.5 months ahead of the internationally used Gregorian calendar (AD/CE).
Named after the historic Emperor Bikramaditya of Ujjain, the calendar was instituted to celebrate regional sovereignty and cultural unity. While neighboring nations primarily use the Gregorian calendar for administration, Nepal proudly maintains Bikram Sambat for government acts, legal statutes, academic calendars, gazetted public holidays, and banking cycles.
The calendar is strictly solar-astronomical. Each month commences on the exact day and minute the Sun transits into a new astrological zodiac constellation (known as a Sankranti). Because the Earth's orbit around the Sun is elliptical rather than circular, the apparent solar speed varies, causing Nepali months to have varying lengths from 29 to 32 days.
This is why a fixed leap-year formula cannot be applied to Bikram Sambat; instead, each year's official month durations are calculated astronomically and announced officially by the Nepal Panchanga Nirnayak Samiti.