proc.time(), which might
not exist on all platforms. It "determines how much time (in seconds)
the currently running R process already consumed". In addition it adds
descriptive names of the returned values.
For more details, see proc.time().
"processTime"(since=NULL, units=c("seconds", "milliseconds", "minutes", "hours", "days"), fmtstr=NULL, ...)character string specifying the unit of the
returned values.sprintf().proc.time().
system.time().
gc.time().