[Rd] Understanding POSIXct creation on different OSes.

[Dirk Eddelbuettel]

On 11 April 2016 at 14:48, Dirk Eddelbuettel wrote:
| 
| 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.

And bumping it a second time.  If nobody replies I will carry it over to Bugzilla.

It is still a "misfeature" with behaviour which varies across OSs which is
rarely the desired outcome.  I'd still love to hear some comments.

Dirk

| 
| 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 at r-project.org mailing list
| | https://stat.ethz.ch/mailman/listinfo/r-devel
| -- 
| http://dirk.eddelbuettel.com | @eddelbuettel | edd at debian.org
| 
| ______________________________________________
| R-devel at r-project.org mailing list
| https://stat.ethz.ch/mailman/listinfo/r-devel
-- 
http://dirk.eddelbuettel.com | @eddelbuettel | edd at debian.org