Tweak the input value for date_modified

Turns out the value used here doesn't matter, because it will always
update to the current time. It just needs to have a changed prop in
order for the save to execute.

plugins/woocommerce/includes/wc-order-functions.php

@@ -660,7 +660,7 @@ function wc_create_refund( $args = array() ) {
 			}
 		}
 
-		$order->set_date_modified( $refund->get_date_created() );
+		$order->set_date_modified( time() );
 		$order->save();
 
 		do_action( 'woocommerce_refund_created', $refund->get_id(), $args );