F-Droid is an installable catalogue of FOSS (Free and Open Source Software) applications for the Android platform. The client makes it easy to browse, install, and keep track of updates on your device.
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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
https://apt.izzysoft.de/fdroid/index/apk/cyou.sk5s.app.countdate
I download it occasionally for random events. It works as it should. Not sure why it requests network permissions. I deny it any privileges and it works as one thinks it should