The following was derived from a forum posting at
http://platon.sk/forum/projects/viewtopic.php?p=607#607

1. Set the invocation to the trigger file

// set the trigger on this table

$opts['triggers']['update']['before']= 'mfg-resources/triggers/date_validate.inc.php';

$opts['triggers']['insert']['before']= 'mfg-resources/triggers/date_validate.inc.php';

2. Contents of trigger file (date_validate.inc.php)

<?php

function valid_date($s)
{
   // Date validation: YYYYMMDD or YYYY/MM/DD or YYYY-MM-DD or YYYY.MM.DD
   if(strpos($s, '-')){
      $token = '-';
   }elseif(strpos($s, '/')){
      $token = '/';
   }elseif(strpos($s, '.')){
      $token = '.';
   }else{
      if(strlen($s) == 8){
         if(checkdate(substr($s, 4, 2),substr($s, 6, 2),substr($s, 0, 4))){
            return true;
         }
      }
   }
   if(empty($token)){
      return false;
   }else{
      $get_date = explode($token, $s);
      $yr = @$get_date[0];
      $mo = @$get_date[1];
      $da = @$get_date[2];
      if($yr > 2037 || $yr < 1900 || !is_numeric($da) || !is_numeric($mo) || !is_numeric($yr) || !checkdate($mo,$da,$yr)) {
         return false;
      }
      return true;
   }
};


$field_name = 'my_date_field';

if(!valid_date($newvals[$field_name])){

   die('<h3> $input_date is not a valid date! </h3>');

}

?>