Elgg uses only a site-wide timezone which is supposed to be the timezone also set and used on the server. The timezone to be used is asked for and set during installation. You can also set it in elgg-config/settings.php.

If you want the users to be able to define their own timezone, you would have to add a user setting in your plugin. Then you would also have to deal with the conversion between the timezones in your plugin (maybe handle it in the server timezone internally and in the database and display maybe only the scheduled time (additionally) for each user specifically). If you would offer the scheduled time both in server time and user time it might also be less confusing because all the other time outputs (e.g. of postings) would still all be in server time.

Thank you for your reply, iionly.

I wrote a plugin (my first completed) for this. I hope others can use it too.

Kind regards,