Content exists in Drupal but unseen in PHPMYadmin?
First of all allow me to say that you should not delete from db directly unless you are really aware of drupal's architecture and how the db tables are organized. (basically have in mind drupal store information for an entity in multiple tables not only one.)

Secondly have in mind in case you are looking through node table for a node created prior 2013-01-01 m the actual date is saved as unix timestamp (1357020000)

Thirdly allow me to recommend you VBO (View Bulk Operation) module for the exact scenario you are looking for , it will let you bulk delete, unpublish, publish (even has an API for custom bulk operations). I would create a simple view accesible for admins only, exposed filters for node creation date and some basic bulk operation. Please consider using this