Drupal 7, Rules, Convert text to date and set a date field
field_get_items() returns an array of items – try strtotime('m-d-Y', $mydate[0]['value']);. Also, it looks like you have an extra ( on your field_get_items call: 

field_get_items((

I would recommend installing the devel module, then you can add this code anywhere you'd like and get the results printed in your messages area for quick debugging and to see what data is available to you and how it's nested:

$mydate = field_get_items($entity_type, $entity, 'field_textdate');
dpm($mydate);