To cite in publications use:

Therneau T, Lumley T, Halvorsen K, Hornik K (2020). date: Functions for handling dates. R package version 1.2-39. S original by Terry Therneau, R port by Thomas Lumley, Kjetil Halvorsen, and Kurt Hornik., https://CRAN.R-project.org/package=date.

Corresponding BibTeX entry:

  @Manual{,
    title = {date: Functions for handling dates},
    author = {Terry Therneau and Thomas Lumley and Kjetil Halvorsen and
      Kurt Hornik},
    year = {2020},
    note = {R package version 1.2-39. S original by Terry Therneau, R
      port by Thomas Lumley, Kjetil Halvorsen, and Kurt Hornik.},
    url = {https://CRAN.R-project.org/package=date},
  }