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/.
Roger S. Bivand, Edzer Pebesma, Virgilio Gomez-Rubio, 2013. Applied spatial data analysis with R, Second edition. Springer, NY. http://www.asdar-book.org/

Corresponding BibTeX entries:

  @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/},
  }
  @Book{,
    author = {Roger S. Bivand and Edzer Pebesma and Virgilio
      Gomez-Rubio},
    title = {Applied spatial data analysis with {R}, Second edition},
    year = {2013},
    publisher = {Springer, NY},
    url = {http://www.asdar-book.org/},
  }