You have done well to notice the way that time is used in this chaotic narrative full of turmoil, which of course mirrors the disorientation of the soldiers and particularly of Jimmy Cross, who feels so responsible for the death of Lavender. We can helpfully group the different time periods into four separate groups: before the war and their arrival in Vietnam, before Lavender's death, Lavender's death and after Lavender's death. Of course, memories and flashbacks intrude, and Jimmy Cross partly blames his "love for Martha" for Lavender's death.
Let us note how flashbacks intrude into the narrative:
Lieutenant Cross remembered touching that left knee. A dark theatre, he remembered, and the movie was Bonnie and Clyde, and Martha wore a tweed skirt, and during the final scene, when he touched her knee, she turned and looked at him in a sad, sober way that made him pull his hand back, but he would always remember the feel of the tweed skirt and the knee beneath it and the sound of the gunfire that killed Bonnie and Clyde.
These memories and flashbacks give more chaos to time and, of course, become another thing to carry, along with the guilt that Jimmy Cross will bear because of the way that he blames himself for the death of Lavender. We are presented with an image of soldiers impossibly weighted down by both physical belongings and also their mental and psychological baggage.