I was just curious of the proper way to set the default value to NONE when creating the date field via PHP code as follows. I have tried setting the default_value parameter to NULL and none without success:

<?php
$fields_array = array(
        array(
            'field' => array(
                'field_name' => 'field_test_date',
                'type' => 'datetime',
                'cardinality' => 1,
                'label' => 'Calendar',
                'settings' => array(),
            ),
            'instance' => array(
                'field_name' => 'field_test_date',
                'entity_type' => 'field_collection_item',
                'bundle' => 'field_test_c',
                'label' => 'Calendar',
                'cardinality' => 1,
                'description' => '',
                '#default_value' => NULL,
                'widget' => array(
                    'type' => 'text_textfield',
                    'weight' => 0,
                    'settings' => array('size' => 50),
                ),
                'display' => array(
                    'default' => array(
                        'label' => 'above',
                        'settings' => array(),
                        'weight' => 1,
                    ),
                    'teaser' => array(
                        'label' => 'above',
                        'settings' => array(),
                        'type' => 'hidden',
                    ),
                ),
                'required' => FALSE,
            )
        ),
    );
?>