Anti SQL Injection on dates

Question

I have a problem and do not know how to solve, I use this function to validate the receipt of some variables:

  if (!function_exists("GetSQLValueString")) {
function GetSQLValueString($theValue, $theType, $theDefinedValue = "", $theNotDefinedValue = "") 
{
  if (PHP_VERSION < 6) {
    $theValue = get_magic_quotes_gpc() ? stripslashes($theValue) : $theValue;
  }

  $theValue = function_exists("mysql_real_escape_string") ? mysql_real_escape_string($theValue) : mysql_escape_string($theValue);

  switch ($theType) {
    case "text":
      $theValue = ($theValue != "") ? "'" . $theValue . "'" : "NULL";
      break;    
    case "long":
    case "int":
      $theValue = ($theValue != "") ? intval($theValue) : "NULL";
      break;
    case "double":
      $theValue = ($theValue != "") ? doubleval($theValue) : "NULL";
      break;
    case "date":
      $theValue = ($theValue != "") ? "'" . $theValue . "'" : "NULL";
      break;
    case "defined":
      $theValue = ($theValue != "") ? $theDefinedValue : $theNotDefinedValue;
      break;
  }
  return $theValue;
}
}

The variables are being treated like this:

$IdCategoria = GetSQLValueString($_POST['IdCategoria'],"int");
$IdUnicoop   = GetSQLValueString($_POST['IdUnicoop'], "int");
$Palavra     = GetSQLValueString('%'.$_POST['Palavra'].'%',"text");

I would like to do the treatment for some dates I have, they are being received so:

$DtInicial   = $_POST['DtInicial'];
$DtFinal     = $_POST['DtFinal'];