[R] problem with as.POSIXct and daylight savings time

[Duncan Murdoch]

On 19/07/2009 11:23 AM, Denis Chabot wrote:
> [was "	[R] end of daylight saving time"]
> 
> Hi,
> 
> I got no reply with the previous subject line, probably a bad choice  
> of subject on my part, so here it is again.
> 
> I read from the help on DateTimeClasses and various posts on this list  
> that, quite logically, one needs to specify if DST is active or not  
> when time is between 1 and 2 AM on the first Sunday in November (for  
> North America in recent years).
> 
> This I can do for on date at a time:
> 
> a <- as.POSIXct("2008-11-02 01:30:00", tz="EST5EDT")  # to get  
> automatic use of DST
> b <- as.POSIXct("2008-11-02 01:30:00", tz="EST")  # to tell T this is  
> the second occurrence of 1:30 that day, in ST
> difftime(b,a)
> 
> Time difference of 1 hours
> 
> But why can't I do the following, which appears to be a typical R way  
> of doing things, to handle several date-times at once?
> 
> c <- rep("2008-11-02 01:30:00", 2)
> tzone = c("EST5EDT", "EST")
> 
> as.POSIXct(c, tz=tzone)
> Erreur dans strptime(xx, f <- "%Y-%m-%d %H:%M:%OS", tz = tz) :
>   valeur 'tz' incorrecte
> 
> ???

Objects of the POSIXlt and POSIXct classes don't support multiple time 
zones, so if you specified several time zones on input, how would the 
conversion functions decide which one to use for output?  You'll need to 
write your own wrapper function to make this decision, and do the 
conversions separately for each input timezone.

Why don't those classes support a separate time zone for each entry? 
Presumably because their designer never thought anyone would want to do 
that.

Duncan Murdoch


> 
> Thanks,
> 
> Denis Chabot
> 
> sessionInfo()
> R version 2.9.1 Patched (2009-07-09 r48929)
> x86_64-apple-darwin9.7.0
> 
> locale:
> fr_CA.UTF-8/fr_CA.UTF-8/C/C/fr_CA.UTF-8/fr_CA.UTF-8
> 
> attached base packages:
> [1] stats     graphics  grDevices utils     datasets  methods   base
> 
> loaded via a namespace (and not attached):
> [1] tools_2.9.1
> 
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