Filter the pmpro_getMemberStartdate() function in Paid Memberships Pro. (Requires PMPro v1.7.8 or higher)

custom_pmpro_getMemberStartdate.php

function custom_pmpro_getMemberStartdate($timestamp, $user_id, $level_id) 
{
	global $wpdb;
	
	if(!empty($level_id))
		$sqlQuery = "SELECT UNIX_TIMESTAMP(startdate) FROM $wpdb->pmpro_memberships_users WHERE membership_id IN(" . esc_sql($level_id) . ") AND user_id = '" . esc_sql($user_id) . "' ORDER BY id LIMIT 1";
	else
		$sqlQuery = "SELECT UNIX_TIMESTAMP(startdate) FROM $wpdb->pmpro_memberships_users WHERE user_id = '" . esc_sql($user_id) . "' ORDER BY id LIMIT 1";
	
	$timestamp = $wpdb->get_var($sqlQuery);
	
	return $timestamp;
}
add_action('pmpro_member_startdate','custom_pmpro_getMemberStartdate', 10, 3);