From the Rhythm Nation 1814 Tour Book: "When we were making the track 'Rhythm Nation' we all got excited thinking that it would make a great anthem for dancers everywhere. Then somebody mentioned that Francis Scott-Key [sic] had written the American national anthem in 1814 and because of that, the number began to take on a significance for us. It wasn't until later that we realized 'R' is the 18th letter of the alphabet and 'N' is the 14th letter. What's even more coincidental is that Scott-Key [sic] wrote the song on September 14th and our album was shipping on September 18th even though it had originally been scheduled to come out in August."
The essay in the liner notes of Design of a Decade 1986/1996 also quotes Janet as saying "1814 is also the year women were finally granted education beyond the eighth grade. That was a key fact for me."
(Source: Wikipedia - http://en.wikipedia.org/wiki/Rhythm_Nation_1814)
