At a crime scene, a forensic technician found the body temperature of the victim was 33C at 6:00 p.m. One hour later, the coroner arrived and found the body temperature of the victim fell to 31.5C. The forensic technician determined that the change in the atmospheric temperature could be modelled satisfactorily as 20e^-0.02tin the time window of 3 hours starting at 6:00 p.m. It is known that the body temperature of alive person is 37C. When was the victim murdered?