In 1990 the GDP of China was estimated to be 12 1012 US dollars, whereas that of the USA was reported to be 5.6 1012 US dollars. The two countries' annual rates of growth were estimated to be 9% and 2% respectively, implying that t years after 1990, their GDP should be Ae rt and Be st respectively, where r = 0.09, s= 0.02, and A, B are suitable constants. Please calculate the date when the two nations' GDP should be the same.