How can I change the Date Created for Assets?
The dateCreated column you're referring to is more of a minimum internal audit trail that Craft uses when a new row is inserted into the database (along with dateUpdated).
Is sounds like you want something that references the original time the asset was created, which would be stored in the images EXIF/metadata.  You could write a plugin that read that information: https://craftcms.stackexchange.com/a/8521/57