In the old days, World Clocks were a sort of curiosity. There wasn’t a lot of need for them because most folks communicated with distant friends and colleagues with snail mail and later, I suppose, with email. In any event you didn’t need to know what time it was at the destination unless you were making an almost certainly hideously expensive phone call.
Now, of course, you can instantly message someone—or even call them in many circumstances—with no charge and it’s best to know that it’s not 3 AM where they are. That’s why, today, a world clock is almost a necessity. You can always ask the Duck or whatever you use but if you’re communicating with several people, it’s convenient to have your own world clock with all the appropriate times.
Emacs, as usual, has us covered. There’s the world-clock command that gives you the current time in a few cities but if you’re parochial like me, knowing what time it is in London doesn’t help much if I want to talk to someone in, say Glasgow. Maybe it’s the same time zone; maybe it isn’t. I have no idea. Like most things in Emacs, the cities displayed are configurable but it’d be pretty inconvenient to add cities on the fly.
The other problem is that only the current time is displayed. What if you’re trying to set up a meeting and want to know what time it will be sometime in the future? It would be nice to be able to adjust both the cities and the time displayed.
Álvaro Ramírez had these problems and being Ramírez he wrote a package that solves both of them. You can add cities on the fly and adjust the base time forward and backward. Right now, the package is only on GitHub so you have to get it from his GitHub repository but he’ll probably put it up on Melpa eventually.
This is a great package that makes figuring out what time it is/will be/was in some arbitrary place in the world easy. Take a look. It may be something you didn’t realize you needed.