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.
In the novel, Vito's death occurs roughly ten years after Connie's wedding (pg. 409):
"It was nearly ten years since there had been such a celebration of people in this house, nearly ten years since the wedding of Constanzia Corleone and Carlo Rizzi."
This passage occurs after Vito's funeral, and before his burial.
Also, it says that it had been "more than a year" since Bonasera had accepted the Don's help when Tom called needing his services.
Also, it says Vito is 60 years old at the time of the "Peace Conference" on page 290.
Also, it took "nearly another year" to bring Michael home from Sicily. So, by the time he gets back, Vito is 61.
Now, one thing, Lucy's dialogue with Jules in the book indicates that Sonny died in 1947. Because it is said that a year had passed since his death when she started seeing Jules. They're talking about sex and refer to fellatio, and he says "Nice girls don't do that. Manly men don't do that. Even int he year 1948."
So the books timeline is probably:
August 1945 - Connie's wedding. This is stated outright, late August 1945.
Christmas 1945 - Vito shot.
1946-1947: Don Corleone lingers in recovery
1947: Sonny is killed.
1947 or 1948 - Peace Summit.
1948 or 1949: Michael returns home
1953-1954: The death of Vito
1955: The Massacre
1956: Move to Nevada. It says in the book that "for twelve months [after the Massacre], Michael divided his time equally between his headquarters at the Long Beach Mall, and his new home in Las Vegas. But at the end of that year, he decided to close out the New York operation and sell the houses and property."
Note - the meeting in which Tessio vents about Barzini's people chewing at his territory occurs
after Vito's death in the book, before his burial. He vents to Michael. It appears that the Family's weakness was due more to Vito dying, as Tessio says Barzini had already opened up two new books in the last day in his territory. Also, Michael asks Pete and Sal to stay int he family for a "year", maybe less, and then they can form their own families if they want.
Note also that Connie had reconciled with her brother by this point, and that she was remarried to an Italian man who worked for the Corleone Family as a secretary.
So, I think the novels and film follow the same rough timeline. Coppola just decided to push Vito's death forward to 1955 rather than 1953 or 1954.
