Awesome

NanoClock

Licensed under the GPLv3+.
Housed here on GitHub.

What does it do?

In the spirit of MiniClock, this aims to display a persistent clock on your Kobo's screen, positioned and formatted to your liking.

Unlike the original MiniClock, or my own previous take on it, this no longer relies on things that don't really have anything to do with the screen to detect screen updates, which means it no longer suffers from any timing related mishaps, is somewhat simpler, and should basically transparently bake the clock into every meaningful screen update with no visible lag.
It also no longer requires firmware patching to handle physical buttons (as we no longer even care about input at all).

It's also written entirely in Lua, instead of a mix of shell & C, which should also help make it more efficient.

The magic does require injecting some code into the kernel in order to allow us to directly listen to screen refresh requests. This may affect portability somewhat, although, so far, the current lineup should be supported without issue, although non-mxcfb platforms (i.e., sunxi on Mk. 8) are somewhat quirkier, and may sport slightly different behavior, or fail to support some features.

Installation

Visit the MobileRead thread to download the install package, and simply unpack the ZIP archive to the USB root of your Kobo when it's plugged to a computer. Do NOT try to open the archive and drag/copy stuff manually, just "Extract to" the root of your device, and say yes to replacing existing content if/when applicable (the directory structure & content have to be preserved verbatim, and there are hidden *nix folders in there that your OS may hide from you)! Then, just eject your device safely, and wait for it to reboot after the "update" process.

If all goes well, you should see the clock appear a few seconds after the device finishes booting, according to the default settings. Take a look at the .adds/nanoclock/nanoclock.ini file on your device to set it up to your liking.
NOTE: Most settings are compatible with their MiniClock counterparts, but the file itself has changed format, so do double-check things manually. Changes to the config file should be picked up immediately.

NOTE: On Mk. 8 (e.g., the Elipsa & Sage), you'll have to reboot the device one more time for the clock to show up! Please also note that FW 4.31.19086 appears to have exposed even more sunxi mishaps, so things may be a bit shaky...

NOTE: On Mk. 9 (Libra 2), this will probably unearth a new weird and wondrous kernel bug that may prevent your device from booting (c.f., https://github.com/koreader/koreader/issues/8414 for more details).
You might want to look into a more integrated solution on that device (e.g., NickelClock ;)).

Uninstallation

Just set the uninstall option in the [global] section of the config file to true.

Credits

Written in LuaJIT, uses FBInk via its Lua bindings.
Relies on my Kobo fork of the mxc_epdc_fb_damage kernel module to provide damage tracking, a module that was originally written by @pl-semiotics for the reMarkable.