How to make a countdown clock?

I've been designing a minigame map, and I need to make it so 40 minutes after I press a button which dictates the start of the minigame, a specific command runs. I also need the countdown from 40 minutes to 0 being visible. The seconds do not necessarily need to be visible, but it would also be great if they could.

I don't know much about commands, but I've watched lots of tutorials on how to do this, so I have a little bit of an idea of how this all would work.
However, those tutorials were for versions before 1.13, and it seems like commands have chanhged completely on 1.13 from previous versions, and I couldn't get myself to just understand how some commands are "translated" from those previous versions to current 1.13 and 1.14.

Alternatively, It would also work for me to build a physical, actual countdown clock that has customizable time and an analog display. However, the only one that would work that I found does NOT have any type of download, and I just can't build a giant redstone machine myself (as I am not very good with that either). I would really appreciate if there was a world I could download containing an already working countdown clock for 1.14

However, commands would still do the trick if that option isn't viable.

 

I think you're on the wrong forum, this is for Bukkit Plugin Developement. If you want I can tell you how to do what you wan't with a plugin, but I don't think thats what you're looking for.