Date-time must be a POSIXct, POSIXlt, Date, Period, chron, yearmon, yearqtr,
zoo, zooreg, timeDate, xts, its, ti, jul, timeSeries, and fts
isoyear() returns years according to the ISO 8601 week calendar.
epiyear() returns years according to the epidemilogical week calendars.
year(x) year(x) <- value isoyear(x) epiyear(x)
a date-time object
a numeric object
the years element of x as a decimal number
year does not yet support years before 0 C.E.
x <- ymd("2012-03-26") year(x)#>  2012year(x) <- 2001 year(x) > 1995#>  TRUE