The American Stock Exchange said it plans to extend its trading day by 30 minutes starting Sept. 30, matching a move by the New York Stock Exchange. Meanwhile, an official for the National Assn. of Securities Dealers, which runs the nation's over-the-counter stock market, said it too would match the NYSE's earlier opening. The changes would be the first expansion in trading hours at the exchanges in more than a decade. They extended their closing times to 4 p.m. Eastern time from 3:30 p.m. on Oct. 1, 1974.