I use Tasks.org on Android which does sync with DavX⁵.
There is no "countdown" but it shows due dates in he widget and has notifications.
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Linux is a family of open source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991 by Linus Torvalds. Linux is typically packaged in a Linux distribution (or distro for short).
Distributions include the Linux kernel and supporting system software and libraries, many of which are provided by the GNU Project. Many Linux distributions use the word "Linux" in their name, but the Free Software Foundation uses the name GNU/Linux to emphasize the importance of GNU software, causing some controversy.
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I use Tasks.org on Android which does sync with DavX⁵.
There is no "countdown" but it shows due dates in he widget and has notifications.
Errands app (gtk) has caldav support for tasks.
https://super-productivity.com/
It’s all Electron though, so the Android widget sucks (won’t update way too often). But otherwise should be a perfect fit for you.
Android has no Electron?
I use Vikunja, but you have to self host it, it's not an app you can just start on your PC