The Pennsylvania Department of Revenue is shortening the so-called look-back period for businesses that pay corporate taxes. The program lets companies voluntarily report it when they discover that they owe back taxes. The time period under the state’s Voluntary Disclosure Program had been five years. But as of this month, it’s dropped to three years. An agency spokesman says the shortened time period brings corporate taxpayers in line with the disclosure period for sales and personal income taxpayers. Jeff Johnson says there’s no penalty for companies that come forward of their own volition — or the volition of their tax preparers.