53rd week of the year in R?

The package ISOweek manages ISO 8601 style week numberings, converting to and from Date objects in R. See ISOweek for more. Continuing the example dates above, we first need to modify the formatting a bit. They must be in form yyyy-Www-w rather than yyyy-ww, i.e. 2009-W53-1. The final digit identifies which day of the week to use in identifying the week, in this case it is the Monday. The week number must be two-digit.

library(ISOweek)dates <- c("2009-50", "2009-51", "2009-52", "2009-53", "2010-01", "2010-02")other.dates <- c("2007-53", "2008-53", "2009-53", "2010-53")dates <- sub("(\\d{4}-)(\\d{2})", "\\1W\\2-1", dates)other.dates <- sub("(\\d{4}-)(\\d{2})", "\\1W\\2-1", other.dates)## Check:dates# [1] "2009-W50-1" "2009-W51-1" "2009-W52-1" "2009-W53-1" "2010-W01-1"# [6] "2010-W02-1"(iso.date <- ISOweek2date(dates))             # deal correctly# [1] "2009-12-07" "2009-12-14" "2009-12-21" "2009-12-28" "2010-01-04"# [6] "2010-01-11"(iso.other.date <- ISOweek2date(other.dates)) # also deals with this# [1] "2007-12-31" "2008-12-29" "2009-12-28" "2011-01-03"## Check that back-conversion works:all(date2ISOweek(iso.date) == dates)# [1] TRUE## This does not work for the others, since the 53rd week of## e.g. 2008 is back-converted to the first week of 2009, in## line with the ISO 6801 standard.date2ISOweek(iso.other.date) == other.dates# [1] FALSE FALSE  TRUE FALSE