Time based coding by attending/teaching physician

Can you please clarify the rules for Time based coding(time spent in discussion/counseling) for E/M codes when there may be a mid-level provider or a Fellow/Resident involved as well at this visit. Is the time additive or the time spent by the Billing provider only is to be counted?

re: Time based coding by attending/teaching physician

When billing on time, only time spent by physician may be billed.