San Diego is 10 hours behind Nairobi
There is no standard 8-to-6 overlap. Someone joins early or late.
Because San Diego is 10 hours behind Nairobi, the two offices are essentially never open at the same time. If a live call is essential, aim for the very start or very end of a workday in one city and accept it'll be off-hours for the other. For part of the year the maths changes: San Diego observes daylight saving, Nairobi does not, shifting the overlap by an hour.