Optional date fields give an error when left empty since upgrade to 2.0

I've just upgraded from 2.0-rc6 to 2.0 and I am now getting the following error when trying to preview a content-type (created using CCK) when the, optional, date fields are left blank:

There are errors in Data Collection:
The To date must be greater than the From date.

I didn't have this problem with 2.0-rc6
I'm running PHP 5.2.6

The export of the field follows:

$content[type]  = array (
  'name' => 'Dataset Details',
  'type' => 'dataset_details',
  'description' => 'A set of data gathered as part of a research project.',
  'title_label' => 'Name',
  'body_label' => '',
  'min_word_count' => '0',
  'help' => '<em>This is where the submission guidelines would go .....</em>',
  'node_options' => 
  array (
    'status' => false,
    'promote' => false,
    'sticky' => false,
    'revision' => false,
  ),
  'upload' => '0',
  'old_type' => 'dataset_details',
  'orig_type' => '',
  'module' => 'node',
  'custom' => '1',
  'modified' => '1',
  'locked' => '0',
  'comment' => '0',
  'comment_default_mode' => '4',
  'comment_default_order' => '1',
  'comment_default_per_page' => '50',
  'comment_controls' => '3',
  'comment_anonymous' => 0,
  'comment_subject_field' => '1',
  'comment_preview' => '1',
  'comment_form_location' => '0',
  'composite_enabled' => 0,
  'composite_referenceable_types' => 
  array (
    'article' => false,
    'book' => false,
    'dataset_details' => false,
    'forum' => false,
    'front_page' => false,
    'image' => false,
    'page' => false,
    'topic_overview' => false,
  ),
  'composite_advanced_view' => '--',
  'composite_advanced_view_args' => '',
);
$content[fields]  = array (
  0 => 
  array (
    'label' => 'Data Collection',
    'field_name' => 'field_dataset_collection_dates',
    'type' => 'date',
    'widget_type' => 'date_text',
    'change' => 'Change basic information',
    'weight' => '5',
    'default_value' => 'blank',
    'default_value2' => 'blank',
    'default_value_code' => '',
    'default_value_code2' => '',
    'input_format' => 'j M Y - g:i:sa',
    'input_format_custom' => 'Y',
    'advanced' => 
    array (
      'label_position' => 'above',
      'text_parts' => 
      array (
        'year' => 0,
        'month' => 0,
        'day' => 0,
        'hour' => 0,
        'minute' => 0,
        'second' => 0,
      ),
    ),
    'increment' => 1,
    'year_range' => '-3:+3',
    'label_position' => 'above',
    'text_parts' => 
    array (
    ),
    'description' => 'Enter the dates of the earliest and latest data included in the dataset.',
    'group' => false,
    'required' => 0,
    'multiple' => '0',
    'repeat' => 0,
    'todate' => 'optional',
    'granularity' => 
    array (
      'year' => 'year',
    ),
    'default_format' => 'field_dataset_collection_dates_default',
    'tz_handling' => 'none',
    'timezone_db' => '',
    'op' => 'Save field settings',
    'module' => 'date',
    'widget_module' => 'date',
    'columns' => 
    array (
      'value' => 
      array (
        'type' => 'varchar',
        'length' => 20,
        'not null' => false,
        'sortable' => true,
        'views' => true,
      ),
      'value2' => 
      array (
        'type' => 'varchar',
        'length' => 20,
        'not null' => false,
        'sortable' => true,
        'views' => false,
      ),
    ),
    'display_settings' => 
    array (
      'label' => 
      array (
        'format' => 'above',
        'exclude' => 0,
      ),
      'teaser' => 
      array (
        'format' => 'field_dataset_collection_dates_default',
        'exclude' => 0,
      ),
      'full' => 
      array (
        'format' => 'field_dataset_collection_dates_default',
        'exclude' => 0,
      ),
      4 => 
      array (
        'format' => 'field_dataset_collection_dates_default',
        'exclude' => 0,
      ),
      2 => 
      array (
        'format' => 'field_dataset_collection_dates_default',
        'exclude' => 0,
      ),
      3 => 
      array (
        'format' => 'field_dataset_collection_dates_default',
        'exclude' => 0,
      ),
      'token' => 
      array (
        'format' => 'field_dataset_collection_dates_default',
        'exclude' => 0,
      ),
    ),
  ),
);
$content[extra]  = array (
  'title' => '-5',
  'menu' => '-3',
  'taxonomy' => '-4',
);