To those who believe Rapture will occur in 2011: What happened in 1891 that leads you to believe that ?

I read somewhere that they use a couple pages in Genesis to prove it will happen in 2011. The only time reference I can think of is God giving man 120 years. I thought that referred only to the Flood. If it also refers to 2011, what happened in 1891 that started the 120 year countdown?