Frank Fina wrote in a motion the alleged incident in which prosecutors said Sandusky sexually assaulted the boy in a locker room shower happened Feb. 9, 2001, not March 1, 2002, as the prosecution originally alleged, the Centre Daily Times of State College, Pa., reported Tuesday.
The motion did not say what led to the change but called it a "result of specific and authenticated findings" in the investigation. Fina said state law allows for a change in the date of the offense if it doesn't change the offense.
Sandusky's lead defense attorney, Joe Amendola, is barred form speaking to the media because of a gag order.
Lawyers for two university officials charged with lying to a grand jury and not properly reporting suspected child abuse said the change in the date indicates prosecutors filed charges against them before they knew the facts, the State College (Pa.) News reported.
In a statement Monday, the lawyers for Penn State Athletic Director Tim Curley, who is on leave from the university, and Gary Schultz, who retired from Penn State as the vice president for business, said: "As we stated in our filings on Friday, the Commonwealth charged this case before it knew the facts. Now, it is clear that Mike McQueary [the graduate assistant] was wrong in so adamantly insisting that the incident happened the Friday before Spring Break in 2002."
Curley and Shultz are seeking to have the charges against them dismissed.
Sandusky, a former assistant football coach, faces 52 criminal charges for allegedly sexually abusing 10 boys during a 15-year span.
He has maintained his innocence.