Validate a date field submitted. Fails if date is same as today's date or if one of the date fields are left empty to prevent defaulting to epoch Jan 1,1970 if day value was omitted.

my-pmpro-registration-checks-date-field.php.php

<?php
/**
 * This recipe checks that a date type Register Helper field had values
 * entered.
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
function my_pmpro_registration_checks_birthday( $okay ) {
	// Set your date field name here.
	$birthday = $_REQUEST['birthday'];

	$today = strtotime( date( 'Y-m-d' ), current_time( 'timestamp' ) );
	$year  = date( 'Y', $today );
	$month = date( 'n', $today );
	$day   = date( 'j', $today );

	if ( ( empty( $birthday['y'] ) || $birthday['y'] === $year )
		&& ( empty( $birthday['m'] ) || $birthday['m'] === $month )
		&& ( empty( $birthday['d'] ) || $birthday['d'] === $day )
		) {
		pmpro_setMessage( 'Please fill out the Birthday field.', 'pmpro_error' );
		return false;
	}

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

This was adapted from this recipe: https://gist.github.com/dparker1005/155113d46e4c2a83f41c38b0c4662193