public function dateSub( $column, $expr, $type )
    {
        $type = $this->intervalMap[$type];
        $column = $this->getIdentifier( $column );

        if ( $column != 'NOW()' )
        {
            $column = "CAST( {$column} AS TIMESTAMP )";
        }

        return " {$column} - INTERVAL '{$expr}' {$type} ";
    }