[development] Skipping validation on date field

I have a birthdate date field in a form where I am using #after_build to modify. The modifications to the drop down values are: - add a "select month", "select day", and "select year" options to their respective drop downs - change the range of years in the year drop down This field(s) is not required. However, if I don't select anything in the drop downs and leave each on the "select month/day/year" values, Drupal validation tells me that the selected date is not valid. How can I tell Drupal to skip this validation? Even if I try to validate this field in hook_form_validate it still tells me the data is invalid. Jeff