A 35-year-old woman presents to her primary care office with a 3-month history of dull headaches, insomnia, and irritability. She has difficulty concentrating and says she is disinterested in her favorite activities. She states she has been having nightmares and frequent flashbacks of her time overseas. The patient recently left the military and is currently employed as a nurse at a local hospital. What important historical feature has she provided that distinguishes her symptoms as post-traumatic stress disorder rather than acute stress disorder? Duration Headaches Insomnia Nightmares