fix date validation for adif export

application/models/Adif_data.php

@@ -82,12 +82,12 @@ class adif_data extends CI_Model {
         $this->db->where($this->config->item('table_name').'.station_id', $station_id);
 
         // If date is set, we format the date and add it to the where-statement
-        if ($from != 0) {
+        if ($from) {
             $from = DateTime::createFromFormat('d/m/Y', $from);
             $from = $from->format('Y-m-d');
             $this->db->where("date(".$this->config->item('table_name').".COL_TIME_ON) >= '".$from."'");
         }
-        if ($to != 0) {
+        if ($to) {
             $to = DateTime::createFromFormat('d/m/Y', $to);
             $to = $to->format('Y-m-d');
             $this->db->where("date(".$this->config->item('table_name').".COL_TIME_ON) <= '".$to."'");