Detect if WordPress post is published for the first time

wp-post-publish-first.php

<?php
/*
 * On first publish set a special date in advance in custom field
 */
add_action('transition_post_status','tanc_first_schedule_publish_set',10,3);
function tanc_first_schedule_publish_set($new, $old, $post) {

	// REFERENCE
	// $new = new post status ('publish')
	// $old = old post status ('draft')
	// $post = Post Object ($post->ID)

	// On first publish
	if ($new == 'publish' && $old != 'publish' && isset($post->post_type)) {

		switch ($post->post_type) {
			case 'type_one' :
				update_post_meta( $post->ID, 'type_one_expiry_date', strftime( '%Y-%m-%d', (strftime('%Y-%m-%d', time()) + strtotime('+365 days')) ) );
			break;
			case 'type_two' :
				update_post_meta( $post->ID, 'type_one_expiry_date', strftime( '%Y-%m-%d', (strftime('%Y-%m-%d', time()) + strtotime('+31 days')) ) );
			break;
			default:
				// Silence is golden
			break;
		}

	}

}
?>

Huh, look who just showed up in my Google search results... works perfectly. Thanks @Jonnyauk 👋