week() returns the number of complete seven day periods that have
occurred between the date and January 1st, plus one.
isoweek() returns the week as it would appear in the ISO 8601
system, which uses a reoccurring leap week.
epiweek() is the US CDC version of epidemiological week. It
follows same rules as isoweek() but starts on Sunday. In other parts of
the world the convention is to start epidemiological weeks on Monday,
which is the same as isoweek.
week(x) <- value
a date-time object. Must be a POSIXct, POSIXlt, Date, chron,
yearmon, yearqtr, zoo, zooreg, timeDate, xts, its, ti, jul, timeSeries, or