The actual story begins on about the fourth page of Chapter 1, when Scout tells us that "That was the summer Dill came to us." The story is set in the early- to mid-1930s. We know this because Atticus states that the year is 1935 during his summation in the Tom Robinson trial.
The story first starts in the future, since the first page of the novel is a recollection by the adult Scout. She mentions Jem's injury from long ago, and she tells the reader that she and Jem still seek out Atticus when they need advice. Since the novel was first published in 1960, we can assume that Scout's remembrance occurs in the late 1950s.
The actual story takes place over the course of about 28-30 months (1933-1935). We know this because Dill comes to visit three times during the summer months. Also, Scout tells us early in the novel that Jem is nearly 10 years old; later, Jem has turned 12 (Chapter 12).
Part One takes place over a two year period, while Part Two comprises a five month period.
The narrative of To Kill a Mockingbird begins with the fourteenth paragraph indentation that sets off the sentence "That was the summer Dill came to us."
At this point in the paragraph, there is a switch in tense. This switch is to present tense, the tense that is used in literary works in order to create a sense of immediacy that draws the audience into the narrative.
The narrative covers a period of time that was difficult for all Americans (the Great Depression) as well as one that posed many problems for African Americans (Jim Crow Laws).
To Kill a Mockingbird is a bildungsroman, a novel of maturation. The Finch children develop both physically and mentally. In their development of new and more mature perspectives, which they glean from various occasions, the children learn the true meaning of Atticus's cautionary words--"try to climb into another's skin and walk around in it."