@misc{Grolemund2012,
author = {Grolemund, G. and Wickham, H  },
title= {Package lubridate},
year = {2012},
URL = {http://cran.r-project.org/web/packages/lubridate/index.html},
note = {(Accessed January 16, 2013)},
edition = {1.2.0},
}