To cite package ‘countdown’ in publications use:

  Aden-Buie G (2024). _countdown: A Countdown Timer for HTML
  Presentations, Documents, and Web Apps_. R package version
  0.4.0.9000, , <https://github.com/gadenbuie/countdown>.

A BibTeX entry for LaTeX users is

  @Manual{,
    title = {countdown: A Countdown Timer for HTML Presentations, Documents, and Web
Apps},
    author = {Garrick Aden-Buie},
    year = {2024},
    note = {R package version 0.4.0.9000, 
},
    url = {https://github.com/gadenbuie/countdown},
  }