R help - Sep 2012 - POSIXlt and daylight savings time

I have a data frame that contains dates, but when I use as.POSIXlt() I lose
the hours on all records. I traced this down to a particuar hour which
causes the issue...
> as.POSIXlt('2004-10-31 02:00:00')
[1] "2004-10-31"
> as.POSIXlt('2004-10-31 03:00:00')
[1] "2004-10-31 03:00:00"

How do I tell as.POSIXlt() to ignore daylight savings and just convert to a
time as is? I've read about the 'isdst' but it is still unclear what
to do.

This is a cleaned up date field that I received so adjusting the date itself
is not possible.

Thanks in advance. 



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a) Don't use POSIXlt in data frames... use POSIXct. POSIXlt is like a data
frame of its own, and data frames within data frames lead to surprising results,
to say the least.

b) I have had best luck using Sys.setenv( TZ="Etc/GMT+8" ) for US
Pacific Standard Time as "local" time for the duration of analysis.

c) Use the format argument if you want consistent results.
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pdb <philb at philbrierley.com> wrote:
>I have a data frame that contains dates, but when I use as.POSIXlt() I
>lose
>the hours on all records. I traced this down to a particuar hour which
>causes the issue...
>
>> as.POSIXlt('2004-10-31 02:00:00')
>[1] "2004-10-31"
>> as.POSIXlt('2004-10-31 03:00:00')
>[1] "2004-10-31 03:00:00"
>
>How do I tell as.POSIXlt() to ignore daylight savings and just convert
>to a
>time as is? I've read about the 'isdst' but it is still unclear
what to
>do.
>
>This is a cleaned up date field that I received so adjusting the date
>itself
>is not possible.
>
>Thanks in advance. 
>
>
>
>--
>View this message in context:
>http://r.789695.n4.nabble.com/POSIXlt-and-daylight-savings-time-tp4642253.html
>Sent from the R help mailing list archive at Nabble.com.
>
>______________________________________________
>R-help at r-project.org mailing list
>https://stat.ethz.ch/mailman/listinfo/r-help
>PLEASE do read the posting guide
>http://www.R-project.org/posting-guide.html
>and provide commented, minimal, self-contained, reproducible code.

pdb wrote:
>> as.POSIXlt('2004-10-31 02:00:00')
> [1] "2004-10-31"
>> as.POSIXlt('2004-10-31 03:00:00')
> [1] "2004-10-31 03:00:00"
> 
> How do I tell as.POSIXlt() to ignore daylight savings and just convert to
> a time as is? I've read about the 'isdst' but it is still
unclear what to
> do.
There *was* no 02:00:00 in your local timezone, so you?re trying to specifiy 
a point in time that just didn?t exist. If you really want to do this, use 
the UTC timezone, by setting the ?tz? argument in as.POSIXct/as.POSIXlt to
UTC (or GMT).

-- 
Karl Ove Hufthammer
E-mail: karl at huftis.org
Jabber: huftis at jabber.no