Take a closer look at the [date parameters][1]; you'll need to enter an integer (number), not a string (text). Simply get rid of the single quotes to make it work:

    $args = array(
      'post_type' => 'job',
      'tax_query' => array(  
        array(  
          'taxonomy' => 'location',
          'terms' => 'dallas',
          'field' => 'slug'
        )  
      ),
      'date_query' => array(
        array(
          'year' => 2014,
          'month' => 12
        ),
      )
    );
    
    $jobs = new WP_Query($args);

  [1]: http://codex.wordpress.org/Class_Reference/WP_Query#Date_Parameters