Before revision: If the timing of infection transmission in deference to a person's clinical onset is TA, and the incubation period of a subsequent case is IB, then the clinical onset serial interval is calculated by TA + IB.
After revision: If the typical time from the first person's clinical onset is TA, and the incubation period of a subsequent case is IB, then the clinical onset serial interval is to when they infect another TA + IB.