All Yahoo Calendar Entries appear on iPhone a day earlier at 11pm-11pm All Yahoo Calendar Entries appear on my iPhone / iPad a day earlier at 11pm-11pm.
Problem continues on both iPhone5s, updated to iOS 10.1.1, and iPad. (I have both Yahoo and [paid] Yahoo Small Business Accounts.) Reinserting the correct day/time on  iPhone lasts for seconds before the entry reverts to 11pm a day earlier. Fortunately, the Yahoo calendar remains OK, but it is not practical to attend client meetings with an iPhone/Yahoo incompatibility.
It may be a time zone discrepancy.
Check that your Yahoo calendar timezone matches the timezone on your phone. For Hong Kong it should be UTC+08:00 on all devices. iOS devices may be updating your timezone automatically based on location.

To check the timezone on your device go to Settings > General > Date &amp; Time.