My apologies, it was me who misunderstood. I missed the important word "update"....
on your update page, add a hidden form element. set the value of the hidden element to:
<?php echo (date("Y-m-d h:n:s", strtotime($row_recordsetName['DateColumn'],'+1 year'))); ?>
where recordsetName is the name of your recordset and DateColumn is the name of your data column
in the update behavior, bind the date column to the hidden element.