Yes, it's true that the massacre occurs in 1955--in the film. The novel doesn't state a date for the massacre. I believe that 1955 would be way too far in the future to be realistic. Michael would have been back in the US, understudying Vito, for eight or nine years before he finally took over. And, Vito and Michael were deliberately acting weak, the other families would have totally plundered the Corleones' holdings over that length of time.