Here's another timeline. Michael murders Sollozzo in early '46, right? Kay had "lived like a spinster for almost two years [after Michael left]." So, that means she calls the Corleone house (p.356) in early '48. Mama Corleone tells her that Michael has been back 6 months. So, that means Michael returned to the States in '47 - maybe as early as September, even August. That fits in with SB's timelie above.

Okay. Now we can be fairly certain of the novel timeline which leads me to ask this question (and not for the first time): What sense does it make to supppose that Michael and Vito would wait as long as 8 years to seek revenge? I'll never buy that timeline.