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time zones, daylight saving etc.

5 messages · Carla Meurk, Gabor Grothendieck, Rich FitzJohn +1 more

Carla Meurk
# 
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
Gabor Grothendieck
# 
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:
Rich FitzJohn
# 
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:

  
    

      
      
    

  


  


      

    

    

      
      

        


  

        

      Brian Ripley
    

    

      
      #
    

  


  

      
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:

        

            

      
      
      
    

        

      
      
    

            

      
      
      
    

        

      
      
    

            

      
      
      
    

        

      
      
    

        

  
    

      
      
    

  


  


      

    

    

      
      

        


  

        

      Gabor Grothendieck
    

    

      
      #
    

  


  

      
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

            

      
      
      
    

        
[1] "2005-05-12 09:08:03 Eastern Daylight Time"

            

      
      
      
    

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

2. OK

C:\>set tz=GMT

C:\>start "" "\Program Files\R\rw2010\bin\r.exe"

R : Copyright 2005, The R Foundation for Statistical Computing
Version 2.1.0 Patched (2005-04-18), ISBN 3-900051-07-0

R is free software and comes with ABSOLUTELY NO WARRANTY.
You are welcome to redistribute it under certain conditions.
Type 'license()' or 'licence()' for distribution details.

  Natural language support but running in an English locale

R is a collaborative project with many contributors.
Type 'contributors()' for more information and
'citation()' on how to cite R or R packages in publications.

Type 'demo()' for some demos, 'help()' for on-line help, or
'help.start()' for a HTML browser interface to help.
Type 'q()' to quit R.

            

      
      
      
    

        
[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: