Bumping this up to the front again ... because it exhibits a difference in behaviour of R across OSs. Such a 'feature' may not be desirable.
On 4 April 2016 at 18:00, Arunkumar Srinivasan wrote: | Hello, | | Following Dirk's post here: https://github.com/Rdatatable/data.table/issues/1619 | we would like to clarify if this is the right behaviour, and if so, | the rationale behind it. | | Here's the summary (thanks to Dirk and Joshua): | | Sys.setenv("TZ"="America/Chicago") | dates = as.Date("2016-03-02") + (0:3)*7 # four Wednesdays | # [1] "2016-03-02" "2016-03-09" "2016-03-16" "2016-03-23" | | # on OS X and Windows 10 -- expected result | as.POSIXct(as.POSIXlt(dates), tz = "America/Chicago") | # [1] "2016-03-02 CST" "2016-03-09 CST" "2016-03-16 CDT" "2016-03-23 CDT" | | # On Linux (tested on Ubuntu 14.04) -- not as expected | as.POSIXct(as.POSIXlt(dates), tz = "America/Chicago") | # [1] "2016-03-02 00:00:00 CST" "2016-03-09 00:00:00 CST" "2016-03-16 | 01:00:00 CDT" "2016-03-23 01:00:00 CDT" | | # 'isdst' attribute is identical on OS X / Windows / Ubuntu, as expected | # since dates don't have timezones, as pointed out by @JoshuaUlrich under | # issue #1619. | lt = as.POSIXlt(dates) | lt$isdst | # [1] 0 0 0 0 | | # However, as Dirk points out, setting isdst to -1 on Ubuntu returns | expected results | lt$isdst = -1 | as.POSIXct(lt, tz = "America/Chicago") | # [1] "2016-03-02 CST" "2016-03-09 CST" "2016-03-16 CDT" "2016-03-23 CDT" | | # Note that setting isdst to -1 on OS X / Windows has no effect, i.e., | the result is always as expected. | | As Dirk points out, from ?POSIXlt: | "Where possible the platform limits are detected, and outside the | limits we use our own C code. This uses the offset from GMT in | use either for 1902 (when there was no DST) or that predicted for | one of 2030 to 2037 (chosen so that the likely DST transition days | are Sundays), and uses the alternate (daylight-saving) time zone | only if ‘isdst’ is positive or (if ‘-1’) if DST was predicted to | be in operation in the 2030s on that day." | | It's not clear what this exactly means, and if it is related to the | behaviour shown above. It'd be nice to know if a) this behaviour of | non-identical result is as expected, and if so, b) can we rely on | setting 'isdst' to -1 returning the expected result always? Could someone from R Core please comment, or would you prefer us to file a bug report? Thanks! Dirk | Thank you, | Arun. | | ______________________________________________ | R-devel@r-project.org mailing list | https://stat.ethz.ch/mailman/listinfo/r-devel -- http://dirk.eddelbuettel.com | @eddelbuettel | e...@debian.org ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
