week() returns the number of complete seven day periods that have
occurred between the date and January 1st, plus one.
the week as it would appear in the ISO 8601 system, which uses a reoccurring
epiweek() returns the week as it conventional in
epidemiology. Epiweek follows same rules as
isoweek() but starts on Sunday.
week(x) week(x) <- value isoweek(x) epiweek(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.
a numeric object
the weeks element of x as an integer number
x <- ymd("2012-03-26") week(x)#>  13week(x) <- 1 week(x) <- 54 week(x) > 3#>  FALSE