How do you update post date (year only) in a separate custom field?

So I was first looking for a way to loop through the existing posts, as suggested in Christine’s comment. Based on this answer, I created a basic plugin that runs the loop upon activation.

// Run the loop when the plugin is activated
register_activation_hook(__FILE__, 'update_my_metadata');
function update_my_metadata(){
$args = array(
    'post_type' => 'post', // Only get the posts
    'post_status' => 'publish', // Only the posts that are published
    'posts_per_page'   => -1 // Get every post
);
$posts = get_posts($args);
foreach ( $posts as $post ) {
    // Run a loop and update every meta data
    $month=get_the_date('M');
    $year=get_the_date('Y');
    update_post_meta($post->ID, 'month-field', $month); echo $month;
    update_post_meta($post->ID, 'year-field', $year); echo $year;
 }
}

However, when I inspect post meta fields, both the month and year are only storing empty strings ” instead of the actual date value.

Any suggestions?