Shenzhen is 16 hours ahead of Portland
Best overlap: 4:00 PM to 6:00 PM in Portland is 8:00 AM to 10:00 AM in Shenzhen.
Plan around 4:00 PM to 6:00 PM in Portland, which is 8:00 AM to 10:00 AM in Shenzhen, to catch everyone during office hours. Overlap is at a premium — roughly 2 hours — so claim 4:00 PM in Portland (8:00 AM in Shenzhen) for recurring calls and keep them on time. The date rolls over between them: Shenzhen is on the next day relative to Portland during these hours. For part of the year the maths changes: Portland observes daylight saving, Shenzhen does not, shifting the overlap by an hour.