for between + 1 month and +2 month would be:
<?php if((date("Y-m-d", strtotime($row_WADAclients['StagingDate'])) > date("Y-m-d", strtotime("+1 months"))) && (date("Y-m-d", strtotime($row_WADAclients['StagingDate'])) < date("Y-m-d", strtotime("+2 months")))) { // Show if column... ?>