Astrological science is based on the time and location of strs in your birthdate at a particular time.So which star is in your timezone at the time of birth and form that the sign is decided.
Birth chart is made on the basis of birth time; place and date. Accurate time of birth is needed to get the perfect birth chart.
Birth chart is used to know the reading or predictions for current or future timings. Birth chart are also used for knowing when are possible chances for marriage, how much one will study and or settle down in their career.
There 12 zodiac sign in astrology and zodiac sign are based on birthdate. Each sign has particular time in which sun transits in that sign for particular time span .

For example: Aries

Sun transits in this sign from March 21-April 19.

So those who have their birthdate in or form March 21 to April 19 will have zodiac sign as Aries.

The readings in numerology are based on the numbers. The summation of birthdate is done and single number is derived. Numerology has set of specific numbers which indicate the importance of that number.
As discussed earlier, from the birth chart all this answers can be known.
Astrology is science, predictions are done on the basis of natal/birth chart .One cannot change their future through astrology but one can surely make it better by following some tips and advice or remedies given by expert astrologers.
Sun sign, zodiac sign, moon sign are the different signs in astrology. This signs are also known as Rashi.
An Astrological sign is determined by the Sun's position at the time of birth. A Horoscope is a map of the heavens, showing the exact position of the Sun, Moon, and Planets, calculated using the exact date, hour and location of birth. Horoscope can provide insight into an individual's personality traits.
Astrology recognizes 10 bodies as "planets": The Sun, the Moon, Mercury, Venus, Mars, Jupiter, Saturn, Uranus and, yes, Pluto. Each planet carries a different energy, and the locations of these planets throughout the cosmos can influence what happens in and around us here on Earth.