Two zeros, one Sun
Western astrology uses the tropical zodiac: a sign is thirty degrees of the Sun's path measured from the March equinox. Most Indian astrology uses the sidereal zodiac, measured from a fixed point among the stars. Because the Earth's axis wobbles, the equinox slides backwards about a degree every 72 years — so the two zeros, which coincided roughly two thousand years ago, are now about 24 degrees apart.
That gap is called the ayanamsa, and 24 degrees is most of a sign. It is why so many people discover their Vedic sign is the one before their Western one, and why anybody born in the last few days of a Western sign almost always shifts. The wobble itself is explained here.
Which ayanamsa, and why it matters
There is no single agreed fixed point, which means there is no single agreed ayanamsa. This tool uses Lahiri, also called Chitrapaksha — the value adopted by the Indian government's calendar reform committee in 1955 and the default in most Indian software. Raman, Krishnamurti and Fagan–Bradley differ from it by up to about a degree, which is enough to move a birth near a boundary into the next sign.
A tool that silently picks one and calls the result "your Vedic sign" is hiding the only real choice it made. This one names it on the result, every time.
The Moon matters more here
In Indian practice the sign that gets asked for is usually the Moon's, not the Sun's — the rashi — and finer still, the nakshatra: one of twenty-seven lunar mansions, each 13°20′ wide, each divided again into four padas of 3°20′.
The twenty-seven are older than the twelve signs and are the oldest surviving division of the zodiac anywhere: they are the Moon's stations, one for roughly each day of its journey round the sky. Each has a symbol, a ruling planet in the Vimshottari sequence, and a character, and the tool reports yours with its pada.
Because the Moon crosses a whole mansion in about a day, a birth time genuinely changes the answer here. The time field is optional and the result says plainly when it was left empty, rather than presenting a coin-flip as settled.
What this is not
It is not a Vedic chart. A real one places nine grahas in twelve houses with a dasha system layered over it, and it is a different discipline with a different vocabulary — this page computes three positions from it and names them accurately. It also is not a claim that one zodiac is correct: both systems have answered their own question consistently for two thousand years, and the disagreement between them is a fact about coordinate systems rather than about astrology.