The stock market posted a modest advance today, with prices extending the day's gains in the final hour of trading. But activity overall was relatively subdued as worries about rising interest rates tended to restrain the advance. The stock market has labored for much of this year under the weight of lackluster corporate earnings reports and rising interest rates. Interest rates have been rising in recent days in anticipation of this week's government auction of $30 billion in new securities. Higher rates make yields on short-term securities more competitive with those on stocks and could hurt the corporate earnings outlook. But even as rates edged higher again today, stock prices gained ground and managed to maintain their tiny gains through the day. In the final hour of trading, prices pushed even higher.