Updates the timezone on all WordPress sites to the specified PHP timezone

update_timezone_on_all_sites.php

<?php 
$timezone = 'America/New_York';

$blogs = wp_get_sites();
foreach ($blogs as $blog) {
    switch_to_blog($blog['blog_id']);
    echo 'Currently: ' . get_option('timezone_string') . ' for blog ' . $blog['blog_id'] . '<br />';
    echo (update_option('timezone_string', $timezone) ? 'Updated' : 'Did Not Update') . '<br /><br/>';
    restore_current_blog();
}