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.