Date-time must be a POSIXct, POSIXlt, Date, chron, yearmon, yearqtr, zoo, zooreg, timeDate, xts, its, ti, jul, timeSeries, and fts objects.
date(x) date(x) <- value
a date-time object
an object for which the
the date of x as a Date
date() does not yet support years before 0 C.E.
date() is not defined for Period objects.