Solution
This works:
Click on Insert Record (users) in the Server Behaviors Panel
<?php echo((isset($_POST["Registration_group_Birthdate"]))?date("Y-m-d", strtotime($_POST["Registration_group_Birthdate"])):"") ?>
New:
<input id="Registration_group_Birthdate" name="Registration_group_Birthdate" type="text" value="<?php echo((isset($_GET["invalid"])?ValidatedField("registration","Registration_group_Birthdate"):"")); ?>" class="formTextfield_Medium" tabindex="3" onblur="hideServerError('Registration_group_Birthdate_ServerError')" />