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Automatic time zone conversion

4 messages · Gabor Grothendieck, Simon Blomberg

Simon Blomberg
# 
Dear R-help,

I was trying to convert a date and time record extracted from a fortran 
subroutine I worte and I encounter some problem. The data read in time 
and date in a format like "2000-05-11_01:00:00.0000" in fortran output. 
It is in GMT. I need to convert it to CST (GMT+8). I did the following 
steps.
 > cdate
[1] "2000-05-11_01:00:00.0000\005\003"
# I am not sure why the extra characters at the end but it doesn't 
affect the strptime function so I just ingored it.
 > strptime(cdate,format="%Y-%m-%d_%H:%M:%S")
[1] "2000-05-11 01:00:00"
# In order to incoporate GMT into the record, I use paste function to 
stick it in.
 >as.POSIXct(as.character(strptime(cdate,format="%Y-%m-%d_%H:%M:%S")),tz="GMT")
[1] "2000-05-11 01:00:00 GMT"
#It is easier to just do a arthmatic to convert the timezone and ingore 
this attribute like
 > 
as.POSIXct(as.character(strptime(cdate,format="%Y-%m-%d_%H:%M:%S")),tz="CST")+(8*3600)
[1] "2000-05-11 09:00:00 CST"
I was wondering if there is a simpler method to do this.

Thanks in advance,

Simon
Gabor Grothendieck
# 
Note that even that will not reliably work on all platforms.  The
only values for the tz= argument that reliably work across
platforms are tz = "" and tz = "GMT".  (See RNews 4/1 Help Desk.)
In fact, entering the above code into my machine

	> R.version.string  # Windows XP
	[1] "R version 2.2.0, 2005-10-24"

gives a different answer than on your machine:

	> as.POSIXct(as.character(strptime(cdate,format="%Y-%m-%d_%H:%M:%S")),
	+	tz="CST")+(8*3600)
	[1] "2000-05-11 08:00:00 CST"

Also if by CST you mean Central Standard Time as in Chicago, Houston
and Winnipeg then its not 8 hours from GMT.  See:

	http://www.stacken.kth.se/~kvickers/timezone.html


Could it be that you just want to read it in as GMT but
display it in the current time zone?  If so, try this:

	x <- as.POSIXct(chartr("_", " ", cdate), tz = "GMT")
	attr(x, "tzone") <- NULL
On 12/5/05, simon <sentientc at gmail.com> wrote:
Simon Blomberg
# 
Hi,

Thanks for the help. Your method does work. However, I am not sure
if my R give CST a correct offset to timezone or at least display it 
normally.
 > cdate
[1] "2000-05-11_01:00:00.0000\005\003"
 > format(as.POSIXct(paste(as.character(strptime(cdate,format=
+"%Y-%m-%d_%H:%M:%S")),"GMT")),tz="CST",format="%Y%m%d %H:%M %Z")
[1] "20000510 17:00 CST"
 > as.POSIXct(as.character(strptime(cdate,format="%Y-%m-%d_%H:%M:%S")),
+ tz="CST")+(8*3600)
[1] "2000-05-11 09:00:00 CST"

 > x <- as.POSIXct(chartr("_", " ", cdate), tz = "GMT")
 > attr(x, "tzone") <- NULL
 > x
[1] "2000-05-11 09:00:00 CST"

One thing is strange here. When I tried to find out what the offset is in R.
 > R.version.string
[1] "R version 2.2.0, 2005-10-06"
 > format(Sys.time(),format="%Z %z")
[1] "CST +0000"
While under command line(fedora core 3), my system display a different 
offset.
$date +"%Z %z"
CST +0800

Thanks again for the help and best regard,

Simon
Gabor Grothendieck wrote:

            

      
      
      
    

        

      
      
    

            

      
      
      
    

  
  


  


      

    

    

      
      

        


  

        

      Gabor Grothendieck
    

    

      
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On 12/6/05, simon <sentientc at gmail.com> wrote:

            

      
      
      
    

        
You could try this:

now <- Sys.time()
now - as.POSIXct(format(now, tz = "GMT"))