Barcelona 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 Barcelona.
3:00 PM to 6:00 PM in Shenzhen is when Barcelona is also working, at 8:00 AM to 11:00 AM their time. Barcelona is 7 hours behind Shenzhen. That leaves about 3 hours of common time — enough for a meeting or two, so use it deliberately. Just remember Barcelona changes its clocks for daylight saving and Shenzhen doesn't, so confirm the overlap whenever the seasons turn.