To cite in publications use:

Terry Therneau and Thomas Lumley and Kjetil Halvorsen and Kurt Hornik (2018). date: Functions for handling dates. R package version 1.2-38.

Corresponding BibTeX entry:

  @Manual{,
    title = {date: Functions for handling dates},
    author = {Terry Therneau and Thomas Lumley and Kjetil Halvorsen and
      Kurt Hornik},
    year = {2018},
    note = {R package version 1.2-38. S original by Terry Therneau, R
      port by Thomas Lumley, Kjetil Halvorsen, and Kurt Hornik.},
    url = {https://CRAN.R-project.org/package=date},
  }