Change pmpro_getMemberStartdate to have everyone starting at 6am.

custom_pmpro_getMemberStartdate.php

function custom_pmpro_getMemberStartdate($timestamp, $user_id, $level_id) 
{
	global $wpdb;
	
	//get timestamp
	$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);
	
	//convert to 6am
	$datetime = date("Y-m-d", $timestamp) . " 06:00";
	
	//convert back to timestamp
	$timestamp = strtotime($datetime);
	
	return $timestamp;
}
add_action('pmpro_member_startdate','custom_pmpro_getMemberStartdate', 10, 3);