require("data.table")
dt <- data.table(id=c(1,1,2,2),
                  event=rep(c("start", "end"), times=2),
                  time=c(as.POSIXct(c("2014-01-31 06:05:30",
                                      "2014-01-31 06:45:30",
                                      "2014-01-31 08:10:00",
                                      "2014-01-31 09:30:00"))))
dt$time[2] - dt$time[1]  # in minutes
dt$time[4] - dt$time[3]  # in hours
dt[ , max(time) - min(time), by=id]  # wrong units printed for id 2

dt[ , difftime(max(time), min(time), units="mins"), by=id]  # both in mins
dt[ , difftime(max(time), min(time), units="hours"), by=id]  # both in hours

dt[ , list(c(max(time) - min(time)),attr(max(time) - min(time),"units")), by=id]

dt[ , list(c(d<-max(time) - min(time)), attr(d,"units")), by=id]
   id        V1    V2
1:  1 40.000000  mins
2:  2  1.333333 hours

dt[ , as.numeric( max(time) - min(time), units="hours" ), by=id]

dt[ , difftime(max(time), min(time), units = 'mins'), by = id]
#    id      V1
# 1:  1 40 mins
# 2:  2 80 mins