About date fields, this page
If you start off without a time zone, but then realize that you need one at a later date [...] Perform the switch in the field settings, but once done, execute the following query:
Computer says no. More exactly, computer says
Attempt to update field Date failed: field_sql_storage cannot change the schema for an existing field with data..
Does anyone know how to do this bypassing the interface, like in the db, in drupal 7 ?