I was reading the JPEG Wikipedia article, and found this quote:
The patent expired in December 2007, but Princeton has sued large numbers of companies for "past infringement" of this patent. (Under U.S. patent laws, a patent owner can sue for "past infringement" up to six years before the filing of a lawsuit, so Princeton could theoretically have continued suing companies until December 2013.)
Does this quote basically says that you cannot sue for software patent infringement after 6 years from the patent expiration date, even if the patent infringement happened while the patent was still not expired?