You cannot buy the same stock on the same day in mainland China. Our country implements a T+1 system, which means that if you buy something on the same day, you cannot sell it on the same day. T+1 refers to the part of the stocks you bought that day, which can only be sold after the next day.

Hong Kong, China, is not so complicated. The stocks bought on the same day can be sold on the same day. The money from the stocks sold that day can be used to buy stocks that day.