Hello! I saw your comment and wanted to let you know how to easily get a countdown timer up and running on your wiki.
Then you're ready to use the countdown timer by following the instructions on the Countdown page.
Your first edit on MediaWiki:ImportJS was correct, so I reverted to that. Then I copied the example from the Countdown page and created a countdown on this test page, which is set to January 1, 2018. Copy & paste the code seen on that page and change the date to whatever you need.