The seasons are tied to the 23.5 degree tilt of the Earth's axis and the position of the Earth in it's orbit. Its an astronomical thing. They bear no direct relation to the Gregorian Calendar. Summer and winter commence at the relevant Solstice, when the overhead Sun is at its Northernmost or Southernmost position. Spring and Autumn begin at the instant the overhead Sun crosses the Equator.
The Equinox and Solstice occur at exactly the same point of the Earth's orbit every year, thus the seasons begin at exactly the same instantaneous point in time, wherever you are in the world.
The seasons do not conveniently begin on the first day of a calendar month, as regularly claimed by the weathermen - who change the definition of the seasons to suit themselves because it's somehow convenient for them to start on the first day of the month. The Gregorian Calendar bears little more than an approximation to the Earth's position in its orbit.
By the weathermen's reasoning, the seasonal change occurs at midnight local time on the first of the month, which means for example that Spring begins here in Sydney a full twenty hours before Autumn begins in Hawaii, which is clearly nonsense.
Astronomers are right with their definition of the seasons and they should be telling the weathermen they cannot redefine the already well defined, just to suit their convenience.
The Southern Hemisphere Spring will commence at the Equinox, as the Sun crosses the Equator, which in 2012 will be at 00.44 am AEST on 23rd September - which in more Westerly Longitudes is at the varying times of xx.44 on 22nd September, their local time .