A judge is asking prosecutors to be more specific about when they believe singer R. Kelly (search) engaged in sex acts with an allegedly underage girl.

Judge Vincent Gaughan (search) made it clear that prosecutors have to be more specific than simply saying the crime took place sometime between November 1997 and February 2002, and said he will hold a hearing on the issue on July 20.

After a half-hour meeting with the attorneys, Gaughan suggested that it was not sufficient for prosecutors to say that the 15-minute tape was made sometime in a 51-month-period.

The Cook County State's Attorney's office contends that they should not have to narrow the time period.

"Our burden is to prove the people on the tape are the defendant and the victim and that the victim is under 18," said spokeswoman Marcy Jensen.

Kelly is charged with 14 counts of child pornography. He appeared in court on Friday, but he and his attorney left without commenting to reporters.