Content exists in Drupal but unseen in PHPMYadmin? I have a content type which I enter data to by means of a csv file. Everything works fine as I have multiple content-types set up like this.

However, I'm trying to clean-up the database and remove data that is from 2012 and before.

Within Drupal at the /admin/content/filter url, I set the parameters for the content-type and such as:

node type is Print Data
and is created before 2013-01-01
and node status is published


(note, I just used the code indicator to set-off the options for this post)

Upon return, I have pages of content with each page showing 500+ results. I can delete them from within Drupal, but only at 500 at a time so I went to phpMyadmin but in there, I can only see/find data that is from 2013.

So where is Drupal storing all that other data if not in the database?
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