About this tool
Many people - especially older relatives - don't have a recorded date of birth, but remember the tithi (lunar day) they were born on, since tithis were traditionally used to mark births rather than the Gregorian or even Bikram Sambat date. Combined with a rough month and year, a tithi usually narrows a birth date down to just one or two candidate days.
This tool scans real astronomical tithi data (Sun-Moon longitude, tithi assigned by the tithi prevailing at sunrise in Kathmandu - the standard Panchang convention) for AD 1930-2010, and cross-checks each candidate against the same Patro calendar engine used elsewhere on this site.
A tithi is a lunar day in the Hindu/Nepali Panchang, defined as the time it takes for the angular distance between the Moon and Sun to increase by 12°. There are 30 tithis in a lunar month - 15 in Shukla Paksha (the waxing fortnight, ending in Purnima/full moon) and 15 in Krishna Paksha (the waning fortnight, ending in Amavasya/new moon).
Before civil registration was common, many families recorded a birth by its tithi and rough season rather than an exact calendar date - it was how time was tracked culturally. A specific tithi within a specific paksha only lands on one or two Gregorian dates within any given lunar month, so combined with even a rough month/year, it narrows things down a lot.
Because tithis are slightly shorter than a solar day on average, a tithi can occasionally span two sunrises (called adhik tithi) or get skipped entirely between two sunrises (called kshaya tithi). Near these boundary cases, different Panchangs can genuinely disagree by a day - that's expected, not an error, which is exactly why this tool shows every matching candidate instead of picking just one.
Tithi is computed from the real apparent geocentric longitudes of the Sun and Moon, referenced to sunrise in Kathmandu - the standard convention Nepali Panchangs use. It isn't hand-approximated; the same astronomical method professional Panchang calculators use is used here, for AD 1930 through 2010.
Try narrowing the year (or reducing the ± years tolerance), picking fewer candidate months, or selecting a nearby fixed-date holiday if you remember the birth being close to one - each narrows the list further.