preventing a date form element from displaying 'day', 'month', and 'year' labels?
I think the property you're looking for is #title_display, not #display_title. Also you wouldn't set it to FALSE but one of these:


before
after
invisible
attribute


In your case you'd want to set it to invisible:

$form['element']['#title_display'] = 'invisible';


That will render the label, so your form remains accessible, but hide it from display.