@tusharhero ha, thanks, I forget that Emacs is an OS because I always use it as a text editor :)
But I tried calc and couldn't understand how to use the unit converter. It is not for me 😄
I think I'll try packaging Cuneo, which seems more like what I'm looking for:
https://github.com/heidefinnischen/cuneo
As for a time zone converter, it seems no one has written one for the desktop. Maybe that's somebody's next project.