Discrepancy in the novel

I began reading it two weeks ago, and have honestly not been thrilled. The dialogue is odd at times, Nick is hard to like, and the pace slacks too much, especially during the romantic trysts. The mafia storyline is overdone and confusing as well. I'm also frequently driven to snacking when I come across Condon's numerous food descriptions (okay, I won't fault the guy for that). However, I decided to slog through until yesterday, when I hit a major snag. NOVEL SPOILER AHEAD...

During a flashback sequence we learn that in December of 1959, Joe Diamond traveled to Detroit and learned that he had been selected to kill the president. Yet, 30 pages before this revelation, there is a flashback sequence to January 1960 when Casper Williams traveled to Philly and informed Diamond he had been chosen to kill the president.

Can somebody explain this to me?