It is a wonderful line. It works because it captures the imagination of the reader in seeking to place them in the context of war through the eyes of the soldier. In terms of the timing of day that the line reveals, we have a couple of elements to dissect. The first line of the poem stresses the idea that "dawnings just begun." This helps to bring out that the exposition of the poem represents the morning, a time in which dawn has broken. In this becoming of day, the rising sun is blocked out due to the bombings. This is where the line gains significance. The morning being awakened by the dawning of the sun, the restoration that this image offers, is offset by the bombing and the construction of war. It is in this setting that the soldier sojourns. The poem focuses on the background of the soldier and the contradictions within him about what he is charged to do. Within this realm of questioning, one sees how the bombing has veiled the sun, reflecting a state of being where hope and restoration are obscured by human constructs. It is this image brought back in the ending of the poem, where the sunset, presumably of that day, is what greets the soldier, now wounded who is wishing to only go home prior to death. The veiling of the bombing to open the poem, the blocking of hope, has become confirmed with the poem's ending.