[Rd] Understanding POSIXct creation on different OSes.

[Dirk Eddelbuettel]

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.
| 
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