Literally, references to sunset and time are significant to the plot because it affects when the boy will cut his hand off with the saw and die. Symbolically, these references to time foreshadow the boy’s death.
This unique poem by Robert Frost describes a boy cutting wood. He loses his hand, and then dies soon after. References to the sunset are there to let you know the approximate length of time the boy has been cutting the wood, and for a practical purpose to point out that it is getting darker.
References to the end of the day symbolically reference the boy’s death. His death is foreshadowed by describing the day as “all but done” and referring to sunset. The sun is about to set on the boy’s life.