Just set this up on a brand new test site and the issue persists. Login details in secure note below, please see the sample page.
Try downgrading Cornerstone 1-2 releases and it should start working.
Code (snippet) used in the element controls is this:
'date' => array(
'type' => 'date',
'ui' => array(
'title' => __( 'Date', 'custom-elements' ),
'options' => array(
'choose_format' => false