When using the Edit widget in Web AppBuilder for ArcGIS to add dates to related record of a new feature, the default time value is always set to 30.12.1899.
In ArcMap, set the time zone to none for related table or select any other field as Time Field and then publish the feature service. Use this feature service web map in the Web AppBuilder for ArcGIS. Now the date is empty for the related table date field as well.