Nairobi is 7 hours ahead of Philadelphia
Best overlap: 8:00 AM to 11:00 AM in Philadelphia is 3:00 PM to 6:00 PM in Nairobi.
For a call that suits both sides, look at 8:00 AM to 11:00 AM in Philadelphia / 3:00 PM to 6:00 PM in Nairobi. Around 3 hours of overlap is enough to work comfortably, as long as meetings don't drift to the end of one team's day. Remember that Philadelphia shifts for daylight saving and Nairobi doesn't, so recalculate the overlap after each clock change.