WordPress - convert Gravity Form date format string to a string without extra characters when submitting to a post type. In my situation I wanted to go from yyyy/mm/dd to yyyymmdd for Advanced Custom Fields compatibility

gravity_forms_date_filter.php

<?php

/**
 * Filter GF date and convert to compatible ACF date string
 * $form['id'] is the id of the Gravity Form and $post_data['post_custom_fields']['event_date'] is the custom field
 */

add_filter('gform_post_data', 'custom_update_gf_date', 10, 3);
function custom_update_gf_date($post_data, $form){

	if($form['id'] == 2 && $post_data['post_custom_fields']['event_date']){
		$post_data['post_custom_fields']['event_date'] = preg_replace('/[^0-9,.]/', '', $post_data['post_custom_fields']['event_date']);
	}

	return $post_data;
}