Taxonomy and Date in same query?

Take a closer look at the date parameters; 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);