Here's proof that the Columbine Shooting happened in 1999.

"20 Apr 2004 — Five years ago today, Eric Harris and Dylan Klebold murdered their classmates and teachers at Columbine High School."
[link to www.theguardian.com (secure)]

Also, I personally remember that year was definitely 1999 because I was worrying that the stupid Y2K bug was going to happen that year and I remember feeling guilty and telling myself that I shouldn't worry about a stupid computer bug when real people died in the shooting.