@misc{Therneau2012,
author = {Therneau, T.M. and Lumley, T. and Halvorsen, K. and Hornik, K.},
title= {date: Functions for handling dates. R package},
year = {2012},
URL = {http://cran.r-project.org/web/packages/date/index.html},
note = {(Accessed January 16, 2013)},
edition = {1.2-33},
}