Mike was explicitly said to have been killed on the night of graduation, which was dated May 18, 1982 in the caption at the start. However his headstone gives his date of death as May 29, 1982.
Lilly mentions at the start that the twenty-fifth anniversary of Mike's death was coming up, but Mike was killed in May of 1982, while this episode took place in the fall of 2007, so the twenty-fifth anniversary would have already come and gone months earlier.
Two detectives enter a firing range without hearing protection to speak to the on-duty range officer, then the range officer takes off her hearing protection to talk to them. No police officer would ever enter a firing range without hearing protection or remove their hearing protection while in the range.