How Vedic placements differ from western ones
Vedic astrology uses the sidereal zodiac, which is offset from the tropical zodiac used in western charts by roughly twenty-four degrees. That offset is why your Vedic moon sign is often one sign back from the western sign you already know, and it is not an error in either system.
The moon rather than the sun carries most of the weight in Jyotish, which is why these tools ask for a birth date and, where possible, a birth time.
Nakshatras, dashas and doshas
The twenty-seven nakshatras divide the zodiac into lunar mansions, each with its own ruling planet and character. Dasha calculators map the long planetary periods a life is divided into, and dosha tools check for the specific placements that traditional matchmaking pays attention to.
A dosha result is not a verdict on a marriage. In practice it is one factor among many, and traditional practice includes remedies rather than refusals.
What these tools approximate
Without an exact birth time and place, any moon-based placement is an approximation and we label it as one. For a full chart with houses, you need a time accurate to a few minutes.
The zodiac library covers the western signs if you want to compare the two systems side by side.