Coundown is a Mac OS X screen saver forked from Motivation. You can use it to countdown to any date you want. (If you pick a date in the past, it will count up from that date instead.)
You can configure the date once you picked it in Screen Saver Preferences in System Preferences.
Download the latest release. Unzip and double-click.
If you see a message saying this screen saver doesn't work with this version of OS X and you have 10.11 or higher, just quit System Preferences and relaunch it. The screen savers is pretty buggy when it comes to screen savers written in Swift.
Xcode 10 is required to build Countdown since it's written in Swift 4.2.