Just when US companies are about to report their biggest writedowns, the losses may be the strongest signal yet that it’s time to buy stocks. Companies in the S&P 500 Index are marking down assets at the fastest rate in six years, leaving operating profits 46% higher than net income in the third quarter, a level last seen in 2003 when the previous bull market began. The ballooning gap between net income and operating profit suggests companies are getting rid of their weakest businesses, setting the stage for a recovery in stocks next year. “Trough eaernings tend to coincide with a maximum level of writedowns,” said William Knapp, NY-based investments strategist at MainStay Investments, which manages $25 billion. “You will start to see profitability return once the economy turns, which will probably be in the second half of next year. The market is going to recognize that through price and activity six-plus months ahead of time.”