A young boy was brought to the hospital on August 17 for a stiff jaw, trouble breathing, and a high temperature, which appeared on August 16. The doctor diagnosed tetanus based on a physical exam, as he reportedly injured himself with a rusty nail on August 3. Metronidazole was given to the boy every six to eight hours for 7 days, and after keeping him for 3 more days, he was discharged on August 28. Identify the incubation period for this case of tetanus.