To cite spacetime in publications use:

Edzer Pebesma (2012). spacetime: Spatio-Temporal Data in R. Journal of Statistical Software, 51(7), 1-30. URL http://www.jstatsoft.org/v51/i07/.

Corresponding BibTeX entry:

  @Article{,
    title = {{spacetime}: Spatio-Temporal Data in {R}},
    author = {Edzer Pebesma},
    journal = {Journal of Statistical Software},
    year = {2012},
    volume = {51},
    number = {7},
    pages = {1--30},
    url = {http://www.jstatsoft.org/v51/i07/},
  }