Adding data render field to PHP line

Okay, I'm using woocommerce and trying to get it so the user can add a date (via Types- pref. in MM/DD format) and have it display in the backorder message. Here is the woocommerce code:

if ( $_product->backorders_require_notification() && $_product->is_on_backorder( $cart_item['quantity'] ) )
          echo '<p class="backorder_notification">' . __( 'Available on backorder till DATE ;<br /> will call when it comes in', 'woocommerce' ) . '</p>';

I want to replace 'DATE' with the actual date the user inputted. Is there actually a way to do that? The php file is one of the default woocommerce php files.