[R] what is isdst parameter

[Prof Brian Ripley]

On 19/01/2015 11:51, Vasantha Kumar Kesavan wrote:
> Hi,
>
> Let me know, how to set/unset "isdst" parameter in system/session level.
>
> I could understand the possible values of isdst 0, 1 and -1.
>
> Example:
>
> Sys.setenv(TZ="PST8PDT");
> as.POSIXct("2007-11-04 01:00:00");
>
> when I was executed the above command,
>
> LINUX.X64 -> it was treated as PST value.
> [1] "2007-11-04 01:00:00 PST"
>
> WINDOWS.X64 -> it was treated as PDT value
> [1] "2007-11-04 01:00:00 PDT"
>
> SOLARIS.X64 -> it was treated as PST value.
> [1] "2007-11-04 01:00:00 PST"
>
> In 2007 the Daylight saving time was ended at 2007-11-04 02:00:00, so there
> was two "2007-11-04 01:00:00" values each for Standard and Daylight time.
>
> But I was not able to understand, how R is displaying the datetime as
> PST/PDT.

?as.POSIXct says

      What happens in such cases is OS-specific: one should
      expect the first to be ‘NA’, but the second could be interpreted
      as either BST or GMT (and common OSes give both possible values).

and note you are unfairly blaming R for what OS services do.

>
> Please do the needful.

_You_ please 'do the needful' and read and follow the posting guide.  We 
know you did not as you posted in HTML ....

>
>
> Thanks
> Vasanth
>
> 	[[alternative HTML version deleted]]
>
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PLEASE do.


-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Emeritus Professor of Applied Statistics, University of Oxford
1 South Parks Road, Oxford OX1 3TG, UK