Converting Unix timestamp to wordpress date

I have trouble converting Unix timestamp (i.e. 1473897600) to WordPress friendly date which displays at the date input field. I have a frontpage, post edit panel which is showing Unix timestamp.

I guess this is the code for getting the post’s date;

<?php   $post_to_edit = array();
        $post_to_edit = get_post($_POST['postid']); 

        $date = $_POST[ '_single_date' ];   ?>

And editing and updating the date with following code works fine;

update_post_meta($pid, '_single_date', strtotime($date) ); 

date input field;

<input value="<?php echo get_post_meta($post_to_edit->ID, '_single_date', true); ?>" name="_single_date" />

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