Preferred timezone should be a user preference.

Note that after applying this change it's important to clear preferred timezone
for system preferences, as they have priority over user preferences

product/ERP5/bootstrap/erp5_core/SkinTemplateItem/portal_skins/erp5_core/Preference_viewHtmlStyle.xml

@@ -79,6 +79,7 @@
                     <value>
                       <list>
                         <string>my_preferred_date_order</string>
+                        <string>my_preferred_time_zone</string>
                         <string>my_preferred_html_style_unsaved_form_warning</string>
                         <string>my_preferred_html_style_developper_mode</string>
                         <string>my_preferred_html_style_contextual_help</string>

product/ERP5/bootstrap/erp5_core/SkinTemplateItem/portal_skins/erp5_core/SystemPreference_view.xml

@@ -82,7 +82,6 @@
                     <value>
                       <list>
                         <string>my_priority</string>
-                        <string>my_preferred_time_zone</string>
                         <string>my_preferred_document_conversion_server_url_list</string>
                         <string>my_preferred_document_conversion_server_retry</string>
                         <string>my_preferred_ooodoc_server_timeout</string>