ZoneStamp!
ZoneStamp is a timezone converter for online events. It's simple, modern, and easy to use. ZoneStamp was originally written by Raquel M. Smith and available on https://zonestamp.com
This current version was forked for the Wikimedia community and is maintained by Derk-Jan Hartman and runs at:
https://zonestamp.toolforge.org
If you run online events with an audience located across different timezones, ZoneStamp makes it easy to create a "Stamp" for the date and time the event will be happening, then send the link for that stamp to your audience. When clicked, the ZoneStamp will automatically show the user what time the event will be happening in their timezone.
ZoneStamp is great for emails - instead of listing three different timezones that don't cover your whole audience, just list one and provide the link to the Stamp.
Also, it's free! Thanks for using ZoneStamp!