<?php the_generator( 'export' ); ?>

$gen = '<!-- generator="WordPress/' . get_bloginfo_rss('version')
    . '" created="'. date('Y-m-d H:i') . '" -->';

<channel>
    ...
    <pubDate><?php echo date( 'D, d M Y H:i:s +0000' ); ?></pubDate>

$wp_filename = $sitename . 'wordpress.' . $date . '.xml';

/**
 * Filters the export filename.
 *
 * @since 4.4.0
 *
 * @param string $wp_filename The name of the file for download.
 * @param string $sitename    The site name.
 * @param string $date        Today's date, formatted.
 */
 $filename = apply_filters( 'export_wp_filename', $wp_filename, $sitename, $date );

add_filter( 'export_wp_filename', function( $wp_filename, $sitename, $date )
{
    return sprintf(
        '%swordpress.%s.xml',
        $sitename,
        date( 'Y-m-dTH-i-s' )
    );

}, 10, 3 );

sitename.wordpress.2016-10-22T19-26-07.xml