Clayton Cramer is searching for the basis of th e10 year sunset of the assault weapons ban. He can't find ten years or September 13 in title 18 of the US Code.
Well, I haven't looked there, but the reason the ban sets on the 13th is because the 1994 Crime Bill as signed by Bill Clinton (PL 103-322) states
SEC. 110105. EFFECTIVE DATE.
This subtitle and the amendments made by this subtitle--
(1) shall take effect on the date of the enactment of this Act; and
(2) are repealed effective as of the date that is 10 years after that date.
It passed out of the Senate and was sent to President Bill Clinton on August 25, 1994. He signed it (Bastard!) on September 13, 1994.
I am not sure why it wasn't reprinted in the U.S. Code, but regardless the actual language of the Public Law is controlling.