diff --git a/date_api.module b/date_api.module
index 67d6d63..db47227 100644
--- a/date_api.module
+++ b/date_api.module
@@ -1254,6 +1254,8 @@ function date_convert($date, $from_type, $to_type, $tz = 'UTC') {
         if ($date['ampm'] == 'pm' && $date['hour'] < 12) $date['hour'] += 12;
         if ($date['ampm'] == 'am' && $date['hour'] == 12) $date['hour'] -= 12;
       }
+      // Default missing date parts to NULL
+      $date = $date + array('month' => NULL, 'day' => NULL, 'hour' => NULL, 'minute' => NULL, 'second' => NULL);
       $datetime = date_pad(intval($date['year']), 4) .'-'. date_pad(intval($date['month'])) .
             '-'. date_pad(intval($date['day'])) .' '. date_pad(intval($date['hour'])) .
             ':'. date_pad(intval($date['minute'])) .':'. date_pad(intval($date['second']));