Colombo is 2.5 hours behind Shenzhen
Best overlap: 10:30 AM to 6:30 PM in Shenzhen is 8:00 AM to 4:00 PM in Colombo.
Your shared working hours run 10:30 AM to 6:30 PM in Shenzhen (8:00 AM to 4:00 PM in Colombo). With 8 hours in common, treat scheduling almost as if the two cities were in the same region.