You can do this using the System expression unixtime to get the current time in milliseconds. Then you need to use a little JS script to get the time in milliseconds of the target date.
With those two numbers you can calculate the difference between them, and from the result extract all the different parts of the countdown timer.
This is a little bit more complex than it seems, so I made a little project. Hopefully this will be useful to others aswell.
The project calculates the time remaining until the 1st of January of 2021, but can be adjusted to count down to other dates as well.