The date_default_timezone_set() is already set by the Language object when the login/loguut page is loaded as long as the locale file exists (which file name matches the language ID of your current language, e.g. “en-US.json”) and contains the “time_zone” setting. If you use multi-language, make sure time zone of all locales are set, and the locale files are generated and uploaded to your site.
If your project use Multi-Language, make sure the setting Multi-Language is checked.
Also make sure all language files with matching language IDs for the bolded locales are placed correctly in the source folder, read Making Language Files.