Site Time Zone = field time zone but != database time zone
When entering data into the field, the data entered is assumed to be in the site's time zone. When the data is saved to the database, it is converted to UTC. When retrieved from the database, the data is converted to the Site's time zone for anonymous users or the User's time zone for logged in users when User-configurable time zones is enabled.
More info https://www.drupal.org/node/767182