Real estate tax laws in the United States vary state by state.
However, I do not know of any state that requires the seller to
prorate real estate taxes. Instead the proration of real estate
taxes is local custom and generally written into an agreement to
buy or sell real estate. Contractually, upon agreement of the
parties involved, the real estate taxes are generally prorated so
that whoever owned the real estate during the calendar year pays
for that same portion of the real estate taxes.