I’m half asleep here so correct me if I am wrong here but it looks to me like you are saving the publish date of the post each time you save the post,

$wr_event_end_date = isset($_POST['wr_event_end_date']) ? $_POST['wr_event_end_date'] : '';
$event_end_date = new DateTime($wr_event_end_date);
update_post_meta($post->ID, "event_end_date", $event_end_date->getTimestamp());

This -> $event_end_date->getTimestamp() being the culprit.

So regardless of your variable $wr_event_end_date value being set or not, your meta value is being assigned via getTimestamp() because firstly there is no conditional statement that checks for the existence of your,

$_POST['wr_event_end_date']

which if FALSE will instead revert to '' (no value) which is then passed to your

new DateTime($wr_event_end_date);

Secondly, because

new DateTime( //is now empty here );

your

getTimestamp()

returns the current time stamp of the post.

UPDATE

You should still use the text_date custom field, example:

array(
    'name' => 'Test Date Picker',
    'desc' => 'field description (optional)',
    'id'   => $prefix . 'test_textdate',
    'type' => 'text_date',
),

…instead of using a timestamp especially if you have no reason to use a timestamp other than to convert it back to a regular date format because WordPress has inbuilt functions for handling date and time formatting.

Here is an example;

global $post;
$text = get_post_meta( $post->ID, '_cmb_test_textdate', true );
$text = date('d F Y', $text); 
echo $text;
    //prints 25 September 2012 for example

I’ve made sure to test this library to ensure it is in fact outputting results correctly and it sure is.

UPDATE

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