Make it so that users can still log in even if they have a bogus timezone stored in user_properties. Currently HHVM systems won't even let them log in due to unhandled exceptions; Zend PHP systems will let them interact with the wiki but the server log will get spewed with PHP warnings.
User:Qian.Nivan cannot login at fawiki due to "Unknown or bad timezone"
Reported at https://meta.wikimedia.org/wiki/Wikimedia_Forum#Please_Help (permalink). Attempts to log in at fawiki throws an exception. Initially I thought this was T119736 but https://meta.wikimedia.org/wiki/Special:CentralAuth/Qian.Nivan works okay and @Tgr said that the log says authentication succeeded but there are SessionManager errors.