In a previous post, we took a look at how to collect a customized date in Drupal. You may have noticed that there are warnings that the Day field are undefined, and that Drupal returns a form error saying the 'specified date is invalid'. This is because we are not passing a day part. In most cases, you would, but using the previous example again of submitting a credit card expiration date, you don't need it. So, how can we solve this problem?