week returns the number of complete seven day periods that have
occured between the date and January 1st, plus one.
isoweek returns the week as it would appear in the ISO 8601 system,
which uses a reoccuring leap week.
Usage
week(x)
week(x) <- value
isoweek(x)
Arguments
x
a date-time object. Must be a POSIXct, POSIXlt, Date, chron,
yearmon, yearqtr, zoo, zooreg, timeDate, xts, its, ti, jul, timeSeries, or
fts object.