In To Kill a Mockingbird, how many years have passed from the start to Tom Robinson's death, and why is this significant?

Assuming you don't count the opening page of the novel, in which Scout reflects upon her past in adult retrospective many years in the future, To Kill a Mockingbird takes place over a period of less than two-and-one-half years. The story begins during the summer before Scout first enters the first grade (in 1933), and it ends on Halloween night during third grade (in 1935). We know the exact years because Atticus tells us during the trial of Tom Robinson that it is "in this year of grace, 1935..." So, the novel takes place between June 1933 and late October 1935--a period of about 28 months. The time frame from the beginning of the story until Tom's death--which occurs in late August 1935 ("August was on the brink of September") is about 26 months. The time frame of two-plus years is primarily significant because both Jem and Scout have matured greatly during this relatively short period: Jem has entered high school, is entertaining a football career, and is growing chest hair; Scout has settled down in school, no longer fights every boy who angers her, is engaged to Dill, and is even showing ladylike tendencies.