here you go
<?php echo(date("Y-m-d h:i:s", mktime(date("h", strtotime($subscriptionDate)), date("i", strtotime($subscriptionDate)), date("s", strtotime($subscriptionDate)), date("m", strtotime($subscriptionDate)), date("d",strtotime($subscriptionDate))+90, date("Y", strtotime($subscriptionDate))))); ?>