Question on Diffie-Hellman patent

Please excuse if this is a FAQ. I'd been operating under the assumption that the Diffie-Hellman patent (4200770) will expire on April 29, 1997 (17 years after the date of issue). Several people have recently told me it doesn't expire until September, evidently confusing the date of issue with the filing date. However, does anyone know of another patent (Hellman-Merkle, perhaps?) that would hve an impact on software that does D-H key exchange? We've got some planned uses of DH; while it doesn't seem cost effective to pay to license DH at this point, that six-month difference is something I need to plan around if necessary. Thanks, Amanda Walker Senior Software Engineer InterCon Systems Corporation