$ npm install -g simple-obs-countdown

$ simple-obs-countdown <endtime> <file>

• endtime: A date string readable by Date.parse.
  • Depending on your string format and system environment, you may need to quote this value.
• file: A file path to write the countdown output

To use this output in OBS, create a new Text Source and select "Read from file". Browse to find the output file you defined above, style the text as desired, and then it can be used as a countdown overlay.