Oh, you have no idea.
Here’s the one I use. It covers most of the major time zones except South Asia. (If anyone knows what city I should be using to get the main timezone of India, please tell me!) The time zone picker field in the Discourse interface can be a bit finicky so I find it easier to just copy and paste this.
[date=2020-02-20 time=10:00:00 format="dddd DD MMMM, YYYY \at HH:mm" timezone="UTC" timezones="America/Los_Angeles|America/Chicago|America/New_York|Europe/London|Europe/Berlin|Europe/Moscow|Asia/Shanghai|Japan|Australia/Sydney"]
Thursday 20 February, 2020 at 10:00