set the birth date column data type in the database to be date.
the data base will expect the format to be:
yyyy-dd-mm
In the insert record behavior, select the birthdate column, click the lightning bolt next to value and select your birthdate form element, this will add code similar to:
[php]<?php echo((isset($_POST["birthDate"]))?$_POST["birthDate"]:"") ?>
you can convert the entered date to the format the database is expecting by changing that to:
[php<?php echo((isset($_POST["birthDate"]))?date("Y-m-d", strtotime($_POST["birthDate"])):"") ?>