Date-time must be a POSIXct, POSIXlt, Date, Period, chron, yearmon, yearqtr,
zoo, zooreg, timeDate, xts, its, ti, jul, timeSeries, and fts
objects.
isoyear()
returns years according to the ISO 8601 week calendar.
epiyear()
returns years according to the epidemilogical week calendars.