To cite in publications use:

Terry Therneau and Thomas Lumley and Kjetil Halvorsen and Kurt Hornik (2014). date: Functions for handling dates. R package version 1.2-34.

Corresponding BibTeX entry:

  @Manual{,
    title = {date: Functions for handling dates},
    author = {Terry Therneau and Thomas Lumley and Kjetil Halvorsen and
      Kurt Hornik},
    year = {2014},
    note = {R package version 1.2-34. S original by Terry Therneau, R
      port by Thomas Lumley, Kjetil Halvorsen, and Kurt Hornik.},
    url = {http://CRAN.R-project.org/package=date},
  }