So this means in the Server Behaviors panel; double click insert record(users); the Web Assist: Insert Single Record dialog box opens; select binding tab; select UserBirthdate column;<?php echo ((isset($_POST["Registration_group_Birthdate"]))?$_POST["Registration_group_Birthdate"]:""); ?>
is what is already in the value lightening bolt text box....

so change that code to:
<?php echo((isset($_POST["Registration_group_Birthdate"]))?date("Y-m-d", strtotime($_POST["Registration_group_Birthdate"])):"") ?>

