A utility function to allow child models to use the type of
date/time format that they prefer. This is primarily used for
setting created_at and updated_at values, but can be used
by inheriting classes.

The available time formats are:
- 'int'      - Stores the date as an integer timestamp
- 'datetime' - Stores the data in the SQL datetime format
- 'date'     - Stores the date (only) in the SQL date format.

@param integer $userData An optional PHP timestamp to be converted.

@return mixed