What is the machine name for the Date Posted?
If you are talking about the 'ID' of the Content: Post date in Views, it is: created

You can find this under the Theme: Information link under the 'Advanced' settings block (in D7/Views 3) or the 'Basic settings' block (in D6/Views 2).  After clicking that link, find the name of the field and find the ID in parenthesis next to the field name.

Example: In this case you should find Field Content: Post date (ID: created)