Antonio D.

Daniil Zhitnitskii

This plugin provides network administrators ability to set same time zone over all blogs in multisite installation in one click, and also it restricts changes of timezone from local blogs settings.
*Based on: https://premium.wpmudev.org/forums/topic/set-default-timezone-for-new-blogs-in-the-network
Only works with multisite installation!
Gutenberg: compatible!
<h3>Disclaimers</h3>
The Simple default timezone plugin is supplied “as is” and all use is at your own risk.

Simple default timezone

As Super Administrator, you can set the (default) timezone for all blogs.

andy

This Plugin allows you to display time and date from all timezones. The setup page generates the shortcode you like. Just copy & paste this code on every place you want. The time is adjusted to the server time. 
Formats: 
 * Time (without seconds) 
 * Time 
 * Date (local format) 
 * Date 
 * Daten and Time

Saan World Clock

This Plugin display date and time of all timezones (Server time adjusted). Easy to use with shortcodes.

Simple default timezone vs Saan World Clock