"Date" representing calendar dates.
"summary"(object, digits = 12, ...)  It is intended that the date should be an integer, but this is not
  enforced in the internal representation.  Fractional days will be
  ignored when printing.  It is possible to produce fractional days via
  the mean method or by adding or subtracting (see
  Ops.Date).
  The print methods respect options("max.print").
Sys.Date for the current date.  Ops.Date for operators on "Date" objects.
  format.Date for conversion to and from character strings.
  axis.Date and
  hist.Date for plotting.
  weekdays for convenience extraction functions.
  seq.Date, cut.Date,
  round.Date for utility operations.
  DateTimeClasses for date-time classes.