To the best of my knowledge I don’t believe there is an exact date for it. In the 11k report it is placed between papers dated 2/2/98 and 6/8/98. Whether that’s coincidence or not I’m unsure. He does mention in the letter that he has 5th period off with her, but the only school schedule I was able to find on him was year 1998-1999 (which doesn’t look like having a period off). Someone else on here may know of the 1997-1998 schedule for him to help pinpoint if he had 5th period off his junior year or his senior year (in case I’m reading his senior schedule wrong). He also states his locker is near the library and includes the locker number 837. Not sure if Columbine had students keep the same locker every single year or not. My school did not, so maybe if they were assigned new lockers every year we can figure out which year he had locker number 837 to help narrow down the date of this letter.