A 38-year-old female emergency medical technician presents 18 days after directly witnessing a mass casualty incident. She reports intrusive distressing memories of the event, recurrent nightmares, severe psychological distress when exposed to ambulance sirens, persistent inability to experience positive emotions, an exaggerated startle response, hypervigilance, difficulty concentrating, and avoidance of news coverage and conversations about the incident. She also reports a sense of altered reality about her surroundings since the event and difficulty remembering key aspects of the incident. She was previously high-functioning with no psychiatric history. Her symptoms began within hours of the event and have persisted without improvement. Which temporal diagnostic consideration is most important?