Hemingway's "slice of life" story takes place in past tense; therefore, it is a flashback (a) as the father recounts what has already transpired. This flashback to the past is then told in chronological order, from beginning to end.
This isolated day has been a momentous day for Schatz because the events of the day have been judged based upon a false supposition and misunderstanding between father and son. For, the father has not been too concerned about his boy's illness since his temperature has remained at 102 degrees Farenheit. However, the son has believed that he is seriously ill, thinking that the 102 degrees has been measured by the metric system of Centigrade. So, when he told his father, "You don't have to stay in here with me...if it bothers you," he has excused his father from watching him die. However, the father's interpretation has been certainly much less serious, as he has known that Schatz's fever has not been high enough to be dangerous. Thinking his boy as being simply light-headed, the father has taken his Irish setter for a walk and to hunt quail.
After his return, the father has realized that he has misinterpreted what Schatz has said; the boy truly has feared death. Sadly, he has let go of any determination through misunderstanding. Thus,
The hold over himself relaxed, too, finally, and the next day it was very slack and he cried very easily at little things that were of no importance.
The father's reflection in hindsight is that the son has taken his condition too seriously and has become distant and detached from life.
i would say that the plot in the " a day's wait" is in flashback. as after the real things known by the father about her sons the flashback of the past that very days behaviour comes to his mind.