To cite x12 in publications use:

Kowarik A, Meraner A, Templ M, Schopfhauser D (2014). “Seasonal Adjustment with the R Packages x12 and x12GUI.” Journal of Statistical Software, 62(2), 1–21. https://www.jstatsoft.org/v62/i02/.

Corresponding BibTeX entry:

  @Article{,
    title = {Seasonal Adjustment with the {R} Packages {x12} and
      {x12GUI}},
    author = {Alexander Kowarik and Angelika Meraner and Matthias Templ
      and Daniel Schopfhauser},
    journal = {Journal of Statistical Software},
    year = {2014},
    volume = {62},
    number = {2},
    pages = {1--21},
    url = {https://www.jstatsoft.org/v62/i02/},
  }