Janma rashi is not your sun sign
The single most common confusion in Indian astrology: why your rashi comes from the moon and needs a birth time.
Two different questions
A western sun sign answers where the sun was on your birthday, and the sun takes a month to cross a sign, so the date alone is enough. A janma rashi answers where the moon was at the moment you were born. The moon crosses a sign in about two and a quarter days, so the same date can carry two different rashis depending on the hour.
Why Vedic astrology prefers the moon
In jyotish the moon stands for the mind, for memory and for emotional temperament, and the whole Vimshottari dasha system — the timeline that tells an astrologer when things happen — is counted from the moon's nakshatra at birth. That is why a genuine reading asks for your birth time and place, not just your date.
The sidereal difference
Indian astrology also uses the sidereal zodiac, correcting for the slow drift of the equinoxes with a value called ayanamsa, most commonly Lahiri. The correction is currently about twenty-four degrees, which is why many people find their Indian sign is one behind the western one they grew up reading.
If you do not know your birth time
A calculation can still be made with noon assumed, and it is usually right. But when the moon was near a sign boundary that day, the rashi and the nakshatra can both shift — an honest astrologer will tell you so and ask you to find the time from a birth certificate or hospital record before relying on the dasha dates.