Adding parsing support for weeks

[jankowtf]

Hi Hadley,

any ideas on why it's not reliably possible to map ISO 8601 week numbers to month of the year numbers? It's also not possible if using the US or UK convention. Tried to outline the problem in this Stackoverflow post. As so often with that sort of stuff I'm suspecting Windows to be the problem here (possibly mixed with a German locale).

Is there some hidden kung-fu in lubridate to hack around this nasty problem? If not, would you kindly consider addressing this issue in one of your next releases?

Best regards,
Janko

[vspinu]

This issue is not well defined. Could you please state precisely the functionality that you would like to see in lubridate with concrete code examples?

[jankowtf]

Hi @vspinu and sorry if I wasn't precise enough.

I'm looking for a way to map a particular week of the year to the correct month of the year and have drafted an example function weeksToMonth below:

weeksToMonth <- function(x, frmt = "%Y-%V-%u", day_of_week = 1) {
  y <- sprintf("%s-%s", x, day_of_week)
  format(as.POSIXct(y, format = frmt), "%Y-%m")
}

I'm aware that in order to do this at all, an algorithm needs the "additional workaround information" of the particular day of the week as otherwise things are completely ambiguous (see argument day_of_week below).

Example dates

Reason: in order to produce inputs of format YYYY-<weeknumber>

(posix <- c(as.POSIXct(c("2015-12-24", "2015-12-31")),
  seq(as.POSIXct("2016-01-01"), length.out = 6, by = "1 week")))
# [1] "2015-12-24 CET" "2015-12-31 CET" "2016-01-01 CET" "2016-01-08 CET"
# [5] "2016-01-15 CET" "2016-01-22 CET" "2016-01-29 CET" "2016-02-05 CET"

Current results (Windows OS and a German locale)

weeksToMonth(format(posix, "%Y-%V"))
# [1] "2015-01" "2015-01" "2016-01" "2016-01" "2016-01" "2016-01" "2016-01" "2016-01"
weeksToMonth(format(posix, "%Y-%U"), frmt = "%Y-%U-%u")
# [1] "2015-12" "2015-12" NA        "2016-01" "2016-01" "2016-01" "2016-01" "2016-02"
weeksToMonth(format(posix, "%Y-%W"), frmt = "%Y-%W-%u")
# [1] "2015-12" "2015-12" NA        "2016-01" "2016-01" "2016-01" "2016-01" "2016-02"

Desired result

weeksToMonth(format(posix, "%Y-%V"))
# [1] "2015-12" "2015-12" "2015-12" "2016-01" "2016-01" "2016-01" "2016-01" "2016-02"
weeksToMonth(format(posix, "%Y-%U"), frmt = "%Y-%U-%u")
# [1] "2015-12" "2015-12" "2016-01" "2016-01" "2016-01" "2016-01" "2016-01" "2016-02"
weeksToMonth(format(posix, "%Y-%W"), frmt = "%Y-%W-%u")
# [1] "2015-12" "2015-12" "2016-01" "2016-01" "2016-01" "2016-01" "2016-01" "2016-02"

As this seems to be a hard one with no easy answer (for Windows OS and German locale, at least), I also posted to r-help which possibly gives you even more background information: http://r.789695.n4.nabble.com/Match-ISO-8601-week-of-year-numbers-to-month-of-year-numbers-on-Windows-with-German-locale-td4728059.html

[vspinu]

In simple words, do you want to be able to parse weeks (US, ISO, EU)? Aka, be able to parse %V, %U and %W formats?

As in:

parse_date_time(c("2015-52", "2015-53"), "YV")
## [1] "2015-12-01 UTC" "2015-12-01 UTC"

Note that R doesn't have partial date-times other than Date object. So the result of this should be either POSIXct or Date.

[jankowtf]

Yes, if there's no "direct approach", I'd like to parse weeks to a POSIX date (including whatever workarounds are necessary to get there; e.g. defining a day of the week) to then express the date as YYYY-mm (or %Y-%m) again.

Note, though, that while this seems to work on MacOS and Ubuntu/Linux at least for %U and %W, it doesn't on Windows (at least for me operating on German locale settings)

[vspinu]

It should be straightforward addition to the internal parser, and should work the same on all platforms. System's strptime is known for buggy/inconsistent parsing of partial date-times.

[jankowtf]

I'm not familiar with the internals of lubridate, but from your answer I take it you're not using whateverstrptime is using internaly but some better/more consistent parser, is that correct? In this case it would be really awesome if you could consider adding parsing weeks to the functionality!

[jolars]

It seems strange to me that the documentation for parse_date_time() suggests that we can use W to parse weeks into dates but that it doesn't work at all:

lubridate::parse_date_time("2015 03", "Y W")
#> [1] "2015-10-29 UTC"

^{Created on 2020-10-29 by the reprex package (v0.3.0)}

^{Created on 2020-10-29 by the reprex package (v0.3.0)}

[cboettig]

Just giving this a 👍 as @hadley suggests in #729

In particular, it is annoying that it as_date() incorrectly parses the ISO 8601 standard week format incorrectly (interpreting week as month) without any warning:

lubridate::as_date("2019-W02-1")
#> [1] "2019-02-01"

^{Created on 2020-12-01 by the reprex package (v0.3.0)}

If we force strptime evaluation I get a different silent error in both ISO week, %V and week %W . (this time using the current month and day instead of the correct one).

lubridate::as_date("2019-W02-1", format = "%Y-W%V-%d")
#> [1] "2019-12-01"
lubridate::as_date("2019-W02-1", format = "%Y-W%W-%d")
#> [1] "2019-12-01"

^{Created on 2020-12-01 by the reprex package (v0.3.0)}