The main difference is the starting point
Western astrology counts from the point of the spring equinox: where the Sun crosses the equator in March, Aries begins. This is the tropical zodiac, tied to the seasons.
Jyotish counts from the stars themselves: Aries begins where Aries is actually seen in the sky. This is the sidereal zodiac. The trouble is that the equinox point slowly drifts — by about a degree every 72 years. Over two thousand years almost 24 degrees have accumulated, and the two systems have drifted apart by nearly a whole sign.
The size of this gap is called the ayanamsha, and it is calculated in different ways. We use the Lahiri ayanamsha — it is accepted in India as the official standard. Right now it equals 23 degrees 51 minutes, and the site recalculates this value on every start and checks it against the reference: if it diverges, the calculations simply won't run.
Why your sign “slips” back by one
Because of those 24 degrees, most people land in the previous sign in an Indian chart. Born on March 25 — Aries in the Western tradition, almost certainly Pisces in Jyotish. The closer your birthday is to the start of a sign, the more likely the shift; in the middle of a sign it also happens, but at the very end — almost never.
There is no mistake in this, and no “correct” answer either. These are two different ways of marking up the same circle. Arguing about which is right is roughly like arguing whether it's correct to measure in meters or in feet.
The Moon instead of the Sun, and 27 mansions instead of 12 signs
The Western tradition builds the conversation around the Sun: “I'm a Libra” means the Sun sign. In Jyotish the Moon is considered the main thing — a description of a person starts with it, and compatibility is built on it. Ask an Indian astrologer who you are, and he'll name your Moon sign.
And there is a layer that Western astrology doesn't have at all: the 27 lunar mansions — nakshatras. The circle is divided not into 12 parts of 30 degrees but into 27 parts of 13 degrees 20 minutes. A nakshatra is a much more precise address than a sign: more than two of them fit inside a single Taurus. Both compatibility and the choice of an auspicious day rest on nakshatras.
Another difference is noticeable right away: classical Jyotish has no Uranus, Neptune or Pluto — they were discovered too late. But it does have Rahu and Ketu, the lunar nodes, and they work as full planets, not as secondary points.
Planetary periods instead of transits
A Western forecast is built mostly on transits: where the planets are moving now relative to your chart. Jyotish relies on something else — dashas, planetary periods laid out from the moment of birth for your whole life ahead.
The system is called Vimshottari and divides 120 years among nine planets: Venus gets 20 years, the Sun 6, the Moon 10, and so on. Which planet your life begins with is determined by the nakshatra of the Moon at birth. Within a big period come sub-periods, and within those, even smaller ones.
The practical difference is this: a transit explains a week, a dasha explains a decade. That's why in our forecast the day and the week are calculated by nakshatras, and the year by the dasha: each scale has its own tool, and substituting one for the other makes no sense.
Which system is more correct
An honest answer: the question is badly put. They answer different questions. Over the last hundred years Western astrology has moved toward psychology — it describes character, inner contradictions, how a person is built inside. Jyotish has remained applied: it talks about timing, about periods, about when to do what.
That's why we calculate both. In a reading you see the natal chart the Western way and the Vedic one side by side, and at the end a short summary in plain language: where they agree and where they argue. They argue less often than you might think: the divergence usually turns out to be a difference of vocabulary, not of substance.