Remove Featured Image based on Date

@Romulus

Since you are using WordPress, I would highly suggest using WordPress’ functionality as much as you can. I would recommend using PHP and creating your own plugin or some kind of script that you can run. There are lots of tutorials out there that will give you the bare-bones implementation for creating a simple WordPress plugin. For now I’ll run you through some helpful WordPress functions that should get you in the right direction.

There is a post from viper007bond.com that shows this snippet of code:

<?php
function last_thirty_days( $where="" ){
    global $wpdb;
    $where .= $wpdb->prepare(" AND post_date > %s", date( 'Y-m-d', strtotime('-30 days'))
    return $where;
}
add_filter( 'posts_where', 'last_thirty_days');
$some_posts = get_posts( array( 'suppress_filters' => false) );
// Important to avoid modifying other queries
remove_filter( 'post_where', 'last_thirty_days');
?>

Though you will have to modify this snippet a bit to fit into your problem, it should suffice as a good starting point for you.

Once you’ve prepared the list of posts (which would be located in $some_posts) you can then loop through each of those posts and run the wordpress function delete_post_meta().

pseudo example:

foreach $post in $some_post loop{
   delete_post_meta($post->ID, '_thumbnail_id');
}

If you don’t want to use filters for some reason you could just return all of your posts using
$allposts = get_posts('numberposts=-1&post_type=post&post_status=any');
loop through all of those results comparing the date to the date you want. Once you’ve found the a post with the date your looking for run the delete_post_meta() on it like I mentioned before.

Hope this helps you!

I highly recommend this wordpress plugin for quickly testing PHP code:
https://wordpress.org/plugins/wordpress-console/
It hasn’t been updated in FOREVER but it still works if you don’t use multisite.