Date-time must be a POSIXct, POSIXlt, Date, chron, yearmon, yearqtr, zoo,
zooreg, timeDate, xts, its, ti, jul, timeSeries, and fts objects. Weeks is
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.