Re: [R] time zones, daylight saving etc.

From: Gabor Grothendieck <ggrothendieck_at_gmail.com>
Date: Thu 12 May 2005 - 23:17:36 EST

I have tried this but on Windows XP R 2.1.0 found I had to set it outside of R prior to starting R.

  1. unsuccessful

> Sys.time()

[1] "2005-05-12 09:08:03 Eastern Daylight Time"
> Sys.putenv(TZ="GMT")
> Sys.time() # no change

[1] "2005-05-12 09:08:12 Eastern Daylight Time"

2. OK

C:\>set tz=GMT

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> Sys.time()

[1] "2005-05-12 13:10:58 GMT" I assume it could be set in .Renviron but it would be nice if one could set it right from within R so that one can write a function that sets it, does processing and then sets it back. Don't know if this is possible.

On 5/12/05, Prof Brian Ripley <ripley@stats.ox.ac.uk> wrote:
> Would it not just be easier to set the timezone to GMT for the duration of
> the calculations? I don't see an OS mentioned here, but on most TZ=GMT
> for the session will do it.
>
> On Thu, 12 May 2005, Rich FitzJohn wrote:
>
> > Hi,
> >
> > seq.dates() in the chron package does not allow creating sequences by
> > minutes, so you'd have to roll your own sequence generator.
> >
> > Looks like the tzone attribute of the times is lost when using min(),
> > max() and seq(). You can apply it back manually, but it does not
> > affect the calculation, since POSIXct times are stored as seconds
> > since 1/1/1970 (?DateTimeClasses).
> >
> > ## These dates/times just span the move from NZDT to NZST:
> > dt.dates <- paste(rep(15:16, c(5,7)), "03", "2003", sep="/")
> > dt.times <- paste(c(19:23, 0:6), "05", sep=":")
> > dt <- paste(dt.dates, dt.times)
> >
> > ## No shift in times, or worrying about daylight savings; appropriate
> > ## iff the device doing the recording was not itself adjusting for
> > ## daylight savings, presumably.
> > datetime <- as.POSIXct(strptime(dt, "%d/%m/%Y %H:%M"), "GMT")
> >
> > ## Create two objects with all the times in your range, one with the
> > ## tzone attribute set back to GMT (to match datetimes), and one
> > ## without this.
> > mindata1 <- mindata2 <- seq(from=min(datetime), to=max(datetime),
> > by="mins")
> > attr(mindata2, "tzone") <- "GMT"
> >
> > fmt <- "%Y %m %d %H %M"
> > ## These both do the matching correctly:
> > match(format(datetime, fmt), format(mindata1, fmt, tz="GMT"))
> > match(format(datetime, fmt), format(mindata2, fmt, tz="GMT"))
> >
> > ## However, the first of these will not, as it gets the timezone all
> > ## wrong, since it's neither specified in the call to format(), or as
> > ## an attribute of the POSIXct object.
> > match(format(datetime, fmt), format(mindata1, fmt))
> > match(format(datetime, fmt), format(mindata2, fmt))
> >
> > ## It is also possible to run match() directly off the POSIXct object,
> > ## but I'm not sure how this will interact with things like leap
> > ## seconds:
> > match(datetime, mindata1)
> >
> > Time zones do my head in, so you probably want to check this all
> > pretty carefully. Looks like there's lots of gotchas (e.g. subsetting
> > a POSIXct object strips the tzone attribute).
> >
> > Cheers,
> > Rich
> >
> > On 5/12/05, Gabor Grothendieck <ggrothendieck@gmail.com> wrote:
> >> You could use the chron package. It represents date times without
> >> using time zones so you can't have this sort of problem.
> >>
> >> On 5/10/05, Carla Meurk <ksm32@student.canterbury.ac.nz> wrote:
> >>> Hi, I have a whole bunch of data, which looks like:
> >>>
> >>> 15/03/2003 10:20 1
> >>> 15/03/2003 10:21 0
> >>> 15/03/2003 12:02 0
> >>> 16/03/2003 06:10 0
> >>> 16/03/2003 06:20 0.5
> >>> 16/03/2003 06:30 0
> >>> 16/03/2003 06:40 0
> >>> 16/03/2003 06:50 0
> >>>
> >>> 18/03/2003 20:10 0.5
> >>> etc. (times given on a 24 hour clock)
> >>>
> >>> and goes on for years. I have some code:
> >>>
> >>> data<-read.table("H:/rainfall_data.txt",h=T)
> >>> library(survival)
> >>> datetime <- as.POSIXct(strptime(paste(data$V1, data$V2), "%d/%m/%Y
> >>> %H:%M"), tz="NZST")
> >>>
> >>> which produces:
> >>>
> >>> [10] "2003-03-13 21:13:00 New Zealand Daylight Time"
> >>> [11] "2003-03-15 13:20:00 New Zealand Daylight Time"
> >>> [12] "2003-03-15 22:20:00 New Zealand Daylight Time"
> >>> [13] "2003-03-15 22:21:00 New Zealand Daylight Time"
> >>> [14] "2003-03-16 00:02:00 New Zealand Daylight Time"
> >>> [15] "2003-03-16 18:10:00 New Zealand Standard Time"
> >>> [16] "2003-03-16 18:20:00 New Zealand Standard Time"
> >>> [17] "2003-03-16 18:30:00 New Zealand Standard Time"
> >>>
> >>> My problem is that "15/03/2003 12:02" has become "16/03/2003 00:02"
> >>> i.e. it is 12 hours behind (as is everything else), but also, I do not
> >>> want to change time zones.
> >>>
> >>> The 12 hour delay is not really a problem just an annoyance, but the
> >>> time zone change is a problem because later on I need to match up data
> >>> by time using
> >>>
> >>> mindata<-seq(from=min(datetime),to=max(datetime),by="mins")
> >>> newdata<-matrix(0,length(mindata),1)
> >>> newdata[match(format.POSIXct(datetime,"%Y %m %d %H
> >>> %M"),format.POSIXct(mindata,"%Y %m %d %H %M"))]<-data$V3
> >>>
> >>> and things go wrong here with matching repeating times/missing times
> >>> around the timezone changes and, my resulting vector is 1 hour shorter
> >>> than my other series. From the R help I see that my OS may be to blame
> >>> but, even if I specify tz="GMT" I still get NZST and NZDT. Can someone
> >>> help?
> >>>
> >>> I hope this all makes sense
> >>>
> >>> Carla
> >>>
> >>> ______________________________________________
> >>> R-help@stat.math.ethz.ch mailing list
> >>> https://stat.ethz.ch/mailman/listinfo/r-help
> >>> PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
> >>>
> >>
> >> ______________________________________________
> >> R-help@stat.math.ethz.ch mailing list
> >> https://stat.ethz.ch/mailman/listinfo/r-help
> >> PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
> >>
> >
> >
> > --
> > Rich FitzJohn
> > rich.fitzjohn <at> gmail.com | http://homepages.paradise.net.nz/richa183
> > You are in a maze of twisty little functions, all alike
> >
> > ______________________________________________
> > R-help@stat.math.ethz.ch mailing list
> > https://stat.ethz.ch/mailman/listinfo/r-help
> > PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
> >
>
> --
> Brian D. Ripley, ripley@stats.ox.ac.uk
> Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
> University of Oxford, Tel: +44 1865 272861 (self)
> 1 South Parks Road, +44 1865 272866 (PA)
> Oxford OX1 3TG, UK Fax: +44 1865 272595
>
> ______________________________________________
> R-help@stat.math.ethz.ch mailing list
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>



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https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html Received on Thu May 12 23:25:02 2005

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