time zones, daylight saving etc.

Works for me on Linux:

Sys.time()

[1] "2005-05-12 10:22:31 PDT"

Sys.putenv(TZ="GMT")
Sys.time()

[1] "2005-05-12 17:22:37 GMT"

I extensively use the reset of TZ to parse times.

-----Original Message-----
From: r-help-bounces at stat.math.ethz.ch
[mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Gabor
Grothendieck
Sent: Thursday, May 12, 2005 6:18 AM
To: Prof Brian Ripley
Cc: Carla Meurk; r-help at stat.math.ethz.ch
Subject: Re: [R] time zones, daylight saving etc.

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 at 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 at 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 at 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

--
Rich FitzJohn
rich.fitzjohn <at> gmail.com   |

http://homepages.paradise.net.nz/richa183

                     You are in a maze of twisty little

functions,

all alike

--
Brian D. Ripley,                  ripley at 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