Set default date and check valid date

check_valid_date.php

<?php

/**
 * PMPro Customization: Register Helper - Add date of birth date picker to checkout
 */

//we have to put everything in a function called on init, so we are sure Register Helper is loaded
function my_pmprorh_init()
{
	//don't break if Register Helper is not loaded
	if(!function_exists( 'pmprorh_add_registration_field' )) {
		return false;
	}
	
	//define the fields
	$fields = array();
	$fields[] = new PMProRH_Field(
		'birthday', // input name, will also be used as meta key
		'date', // type of field
		array(
			'label'     => 'Date Of Birth', // custom field label
			'class'     => 'date', // date class
			'profile'   => true, // show in user profile
			'required'  => true, // make this field required
			// 'addmember' => true, // Uncomment to display in Add Member
			// 'memberslistcsv' => true, // Uncomment to Include in Member List CSV Export
		)
	);

	//add the fields into a new checkout_boxes are of the checkout page
	foreach($fields as $field)
		pmprorh_add_registration_field(
			'checkout_boxes',				// location on checkout page
			$field						// PMProRH_Field object
		);
	//that's it. See the PMPro Register Helper readme for more information and examples.
}
add_action( 'init', 'my_pmprorh_init' );

function set_default_register_helper_date()
{
	//Assumes RH field with key = "birthday"
?>	<script>
	
	jQuery('#birthday_m').val(1);
	jQuery('#birthday_d').val('mm');
	jQuery('#birthday_y').val('yyyy');
	</script>
<?php
}

add_action("pmpro_checkout_after_form", "set_default_register_helper_date");


function custom_pmpro_validate_user_age( $okay ) {

	if( ! $okay ) {
		return $okay; //handle other errors first.
	}
	 

	global $pmpro_msg, $pmpro_msgt;

	$date = $_POST[ 'birthday' ];

	$year = $date[ 'y' ];

	if( $year == 'yyyy' ){
		$pmpro_msg = "Please select a valid date.";
		$pmpro_msgt = "pmpro_error";
		return false;
	}

	return $okay;
}
add_filter( 'pmpro_registration_checks', 'custom_pmpro_validate_user_age' );