R help - Jul 2012 - Undocumented behavior around daylight savings time?

Apologies for the intrusion. I am a lurker on list.

I have been working to convert a digitized signal from a matlab file into R
for analysis and other applications. R.matlab is working fine, and it is
easy to convert the matlab date-time number (days since year 0) into R
date-time numbers (seconds since 1970-01-01).

Unfortunately, when I cast the R date-time number into POSIXct format it
seems to adjust silently by one hour to reflect daylight savings time, but
I have been unable to suppress that behavior. (The problem is that the
matlab date is already in MDT, and I don't want to have to write my own
code to suppress the added hour only when local DST rules apply.)

To use the example of the R date-time number 1340717324, this is the
behavior I observe:
> as.POSIXct(1340717324,origin='1970-01-01')
[1] "2012-06-26 14:28:44 MDT"
> as.POSIXct(1340717324,origin='1970-01-01',tz='')
[1] "2012-06-26 14:28:44 MDT"
>
as.POSIXct(1340717324,origin='1970-01-01',tz='America/Denver')
[1] "2012-06-26 14:28:44 MDT"
> as.POSIXct(1340717324,origin='1970-01-01',tz='MST')
[1] "2012-06-26 13:28:44 MST"
> as.POSIXct(1340717324,origin='1970-01-01',tz='UTC')
[1] "2012-06-26 13:28:44 UTC"
I was ultimately able to solve the problem by casting into and out of a
character string, but that seems risky/error prone.

rdates='%Y-%m-%d %H:%M:%S'
>
as.POSIXct(strptime(as.character(as.POSIXct(1340717324,origin='1970-01-01',tz='UTC')),format=rdates))
[1] "2012-06-26 13:28:44 MDT"

I have read the various help entries and even investigated the lubridates
package, but none indicate why exactly the extra hour is being added or how
to suppress it. (Note that I tried as.POSIXlt with various settings of
isdst, none of which worked).

I'm using "R version 2.13.1 (2011-07-08)" on
"x86_64-apple-darwin9.8.0".



-- 

----------------
Samuel Brown

	[[alternative HTML version deleted]]

Set your default local timezone (at least while converting to POSIXt types:

Sys.setenv(TZ="Etc/GMT+7")
---------------------------------------------------------------------------
Jeff Newmiller                        The     .....       .....  Go Live...
DCN:<jdnewmil at dcn.davis.ca.us>        Basics: ##.#.       ##.#.  Live
Go...
                                      Live:   OO#.. Dead: OO#..  Playing
Research Engineer (Solar/Batteries            O.O#.       #.O#.  with
/Software/Embedded Controllers)               .OO#.       .OO#.  rocks...1k
--------------------------------------------------------------------------- 
Sent from my phone. Please excuse my brevity.

Samuel Brown <samuelbrown at gmail.com> wrote:
>Apologies for the intrusion. I am a lurker on list.
>
>I have been working to convert a digitized signal from a matlab file
>into R
>for analysis and other applications. R.matlab is working fine, and it
>is
>easy to convert the matlab date-time number (days since year 0) into R
>date-time numbers (seconds since 1970-01-01).
>
>Unfortunately, when I cast the R date-time number into POSIXct format
>it
>seems to adjust silently by one hour to reflect daylight savings time,
>but
>I have been unable to suppress that behavior. (The problem is that the
>matlab date is already in MDT, and I don't want to have to write my own
>code to suppress the added hour only when local DST rules apply.)
>
>To use the example of the R date-time number 1340717324, this is the
>behavior I observe:
>
>> as.POSIXct(1340717324,origin='1970-01-01')
>
>[1] "2012-06-26 14:28:44 MDT"
>
>> as.POSIXct(1340717324,origin='1970-01-01',tz='')
>
>[1] "2012-06-26 14:28:44 MDT"
>
>>
as.POSIXct(1340717324,origin='1970-01-01',tz='America/Denver')
>
>[1] "2012-06-26 14:28:44 MDT"
>
>> as.POSIXct(1340717324,origin='1970-01-01',tz='MST')
>
>[1] "2012-06-26 13:28:44 MST"
>
>> as.POSIXct(1340717324,origin='1970-01-01',tz='UTC')
>
>[1] "2012-06-26 13:28:44 UTC"
>I was ultimately able to solve the problem by casting into and out of a
>character string, but that seems risky/error prone.
>
>rdates='%Y-%m-%d %H:%M:%S'
>>
>as.POSIXct(strptime(as.character(as.POSIXct(1340717324,origin='1970-01-01',tz='UTC')),format=rdates))
>[1] "2012-06-26 13:28:44 MDT"
>
>I have read the various help entries and even investigated the
>lubridates
>package, but none indicate why exactly the extra hour is being added or
>how
>to suppress it. (Note that I tried as.POSIXlt with various settings of
>isdst, none of which worked).
>
>I'm using "R version 2.13.1 (2011-07-08)" on
>"x86_64-apple-darwin9.8.0".
>
>
>
>-- 
>
>----------------
>Samuel Brown
>
>	[[alternative HTML version deleted]]
>
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