Adjustment for time zone

I have the module working, using the code below. The problem is, it picks up the time from what I assume is GMT, so events that occur after, say, 6 PM on April 15 will actually appear as if they occur on the following day. Any ideas? Thanks much!

function content_calendar_block(&$calendar, &$date, $op) {
  switch($op) {
    case 'alter':
    $sql = "SELECT content_type_event.field_datetime_value, content_type_event.nid FROM content_type_event;";
 $result = db_query( $sql );
 $dateDates = array();
 while ($node = db_fetch_object( $result))
 { 
 $dateDates[] = node_load ( array( 'nid' => $node->nid));
 }
  
 foreach ($dateDates as $var){
 $thisPath = $var->path;
 $thisDate = substr($var->field_datetime[0]['value'],0,10);
 $realDate = substr($thisDate, 5, 2) . "-" . substr($thisDate, 8,2)."-". substr($thisDate, 0, 4);
  
 if ($date->date == $realDate) {
 
 $date->content = l($date->day, "events/".$thisDate); 
 } 
 } 
 break;
  }
}