https://github.com/sk5s/countdateapp

Get the IzzyOnDroid repo or use Obtainium. There are tons more opensource apps not on F-Droid but released by the Developers themselves.

Izzyondroid just takes these releases.

ProgressBars (A simple timer / countdown app) https://f-droid.org/packages/org.mattvchandler.progressbars/

I used that one

It's not FOSS but you can do this in Tasker easily enough.

edit: I wasn't satisfied with what I said, so here's another approach using things available on F-Droid.

With Termux, Termux:API, and Termux:boot, you can use termux-job-scheduler to run a script to calculate your remaining time to a target date-time (use UNIX timestamp for the calculation and strftime to format it) and then use termux-notification to publish a notification on your system bar. You'd use termux:boot to make sure the script gets (re-)scheduled after each reboot.

Termux is just generally useful for a lot of things. I think its worth the storage to maintain it, and I use it quite a bit myself like I use Linux terminal on the daily.

I also use Tasker quite a bit, and have your specific countdown use case implemented already. It was counting down the number of days remaining to 9am local time of a specific date, and would notify me every morning at 9am. I can share an export if you'd like.

There's also https://github.com/sk5s/countdateapp of you want to go the purely app-based route