This appears to be a bug.
Please
file a bug report and...
1. Give as much information as possible.
2. Provide step-by-step instructions how to reproduce the problem.
3. Provide screenshots and a sample survey.
4. Post the link to the bug here so we can follow the progress.
In the meantime, you can work around the problem by using the "dd mm yyyy" format and add the following script to the question source. This will select the day = 01 option and hide the day dropdown.
<script type="text/javascript" charset="utf-8">
$(document).ready(function(){
$('.date').each(function(i){
$('.day option[value="01"]', this).attr('selected', 'selected');
$('.day', this).hide();
});
});
</script>
