TithiTithi and Bikram Sambat, explained
Nepali dates live in two systems at once: the solar Bikram Sambat calendar and the lunar tithi. Here is what each one measures, where it comes from, and why neither repeats on a fixed AD date.
A tithi is an angle, not a day
A tithi is a lunar day. It is measured by the angle between the Moon and the Sun as seen from Earth. At amavasya (aunsi) the two line up; from then on the Moon gains on the Sun every hour. Each time it gains another 12 degrees, a new tithi begins.
A full circle of 360 degrees holds 30 such steps, so every lunar month has 30 tithis: 15 in shukla paksha, the waxing half that ends at purnima, and 15 in krishna paksha, the waning half that ends at aunsi.
Why a tithi points to an exact moment
Because a tithi is defined by where the Sun and Moon are in the sky, it begins at one instant for the whole planet. When the angle reaches the next multiple of 12 degrees, the new tithi has started in Kathmandu, Sydney and New York alike; only the clock time on the wall differs.
The Moon's orbit is an ellipse, so it moves faster near Earth and slower far from it. A tithi therefore lasts anywhere from about 19 to about 26 hours, and it can start at any hour. It never lines up neatly with a midnight to midnight day.
Which tithi a day belongs to
Patros name each day by the tithi in force at sunrise, the udaya tithi. Sunrise depends on where you stand, so a patro is computed for a place. Tithi uses Kathmandu, as Nepali patros do.
Since a tithi can be shorter than a day, one sometimes begins after a sunrise and ends before the next. It never touches a sunrise, so no day carries its name: a kshaya tithi. Tithi reminds you on the day it begins. A long tithi can span two sunrises, a vriddhi tithi; Tithi uses the first of the two days.
Lunar months and the extra month
Twelve lunar months come to about 354 days, some 11 days short of the solar year. To keep festivals in their seasons, the lunar calendar adds an adhik (extra) month roughly every two and a half to three years: a month during which the Sun does not enter a new rashi.
Nepali patros name months the Purnimanta way, with each month ending at purnima. In a year with an adhik month, Tithi places yearly dates in the regular (nija) month of that name, the way families usually observe them.
Where Bikram Sambat comes from
Bikram Sambat (Vikram Samvat) is named after King Vikramaditya of Ujjain, who by tradition began the era in 57 BCE to mark a victory over the Shakas. Historians note that early inscriptions call the era Krita or Malava, and that Vikramaditya's name was attached to it later. The era itself is old and widely used across South Asia.
A BS year runs about 56 years and 8 months ahead of the AD year: 2083 BS began in April 2026.
Bikram Sambat in Nepal
In Nepal, BS is the official calendar. Its government use dates to 1958 BS (1901 AD), under Prime Minister Chandra Shumsher. Schools, offices, salaries, citizenship papers and most birthdays run on it.
Nepal's BS is a solar calendar. Each month starts at a sankranti, the moment the Sun enters a new sidereal rashi, and the year starts at Mesh sankranti, 1 Baishakh, in mid-April. Earth moves faster at some points in its orbit than others, so months last 29 to 32 days and their lengths change from year to year. That is why a BS date converts to a different AD date each year, and why the converter needs published tables rather than a formula.
Why this matters for reminders
Birthdays are often kept by BS date, while festivals, lunar birthdays and shraddha follow the tithi. Neither repeats on a fixed AD date, so an ordinary calendar's yearly repeat drifts off within a year or two.
Tithi works out each year's BS date, AD date and tithi from the positions of the Sun and Moon (sidereal, Lahiri ayanamsa, at sunrise in Kathmandu), and puts the result in the calendar you already use.