Not sure if I'm doing something wrong but when defining the field as:
$cmb->add_field( array(
'name' => __( 'Test Date Range', 'cmb2' ),
'desc' => __( 'field description (optional)', 'cmb2' ),
'id' => $this->field_prefix . 'date_range',
'type' => 'date_range',
) );
This is what gets saved to the database:
a:2:{s:5:"start";s:6:"9/28/Y";s:3:"end";s:7:"10/31/Y";}
i.e. the year is saved as "Y" instead of the actual year