Frankfurt is 7 hours behind Shenzhen
Best overlap: 3:00 PM to 6:00 PM in Shenzhen is 8:00 AM to 11:00 AM in Frankfurt.
Shenzhen and Frankfurt overlap from 3:00 PM to 6:00 PM (Shenzhen) to the equivalent 8:00 AM to 11:00 AM in Frankfurt — that's your meeting window. The shared window is roughly 3 hours; treat its midpoint as the default and you'll keep both teams happy. A quick caveat — Frankfurt's clocks move for daylight saving and Shenzhen's don't, so the times here are right for only part of the year.