[R] time zones in POSIXt

[Vadim Ogranovich]

Thank you for the lead, Dirk! Indeed this works on my machine too:

> as.POSIXct("2000-05-10 10:15:00",  tz="PST8PDT") -
as.POSIXct("2000-05-10 10:15:00",  tz="GMT")
Time difference of 7 hours


However when I replace POSIXct by POSIXlt it breaks (this looks like a
bug to me):

> as.POSIXlt("2000-05-10 10:15:00",  tz="PST8PDT") -
as.POSIXlt("2000-05-10 10:15:00",  tz="GMT")
Time difference of 0 secs



Now a couple of new questions:
* how could I learn about appropriate names for time zones? For example
I was using "PST" whereas it seems I had to use either "PST8" or
"PST8PDT". Why "PST" was not good? Is it documented anywhere?

* there seems to be no difference betweeen GMT and BST on my machine
though PST8 and PST8PDT are treated properly:

# PST8 is not identical to PST8PDT
> ISOdatetime(2003, seq(12), 1, 10, 0, 0, tz="PST8") - ISOdatetime(2003,
seq(12), 1, 10, 0, 0, tz="PST8PDT")
Time differences of    0,    0,    0,    0, 3600, 3600, 3600, 3600,
3600, 3600,    0,    0 secs

# GMT0 is identical to BST
> ISOdatetime(2003, seq(12), 1, 10, 0, 0, tz="GMT0") - ISOdatetime(2003,
seq(12), 1, 10, 0, 0, tz="BST")
Time differences of 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 secs

Why is such dichotomy?

Thanks,
Vadim






> -----Original Message-----
> From: Dirk Eddelbuettel [mailto:edd at debian.org] 
> Sent: Friday, April 23, 2004 1:52 PM
> To: Vadim Ogranovich
> Cc: R-Help
> Subject: Re: [R] time zones in POSIXt
> 
> 
> 
> On Fri, Apr 23, 2004 at 11:30:19AM -0700, Vadim Ogranovich wrote:
> > Hi,
> >  
> > I have two data sources. One records time in PST time zone, 
> the other 
> > in GMT. I want to compute the difference between the two, but don't 
> > see how. Here is an example where I compute time difference between 
> > identical times each (meant to be) relative to its time zone.
> >  
> > > as.POSIXlt("2000-05-10 10:15:00",  "PST") -  
> as.POSIXlt("2000-05-10
> > 10:15:00",  "GMT")
> > Time difference of 0 secs
> > 
> > I was expecting to see 8hrs (which is the time difference between 
> > London and San-Francisco). Why is it so and what is the 
> correct way of 
> > doing it?
> 
> Seems to work with POSIXct in 1.8.1 and 1.9.0:
> 
> > as.POSIXct("2000-05-10 10:15:00",  "PST8PDT") -  
> > as.POSIXct("2000-05-10
> 10:15:00", tz="UTC")
> Time difference of 7 hours
> 
> 
> Dirk
> 
> >  
> >  
> > I use R-1.8.1 on RH-7.3.
> >  
> > Thanks,
> > Vadim
> >  
> >  
> > 
> > 	[[alternative HTML version deleted]]
> > 
> > ______________________________________________
> > R-help at stat.math.ethz.ch mailing list 
> > https://www.stat.math.ethz.ch/mailman/listinfo/r-help
> > PLEASE do read the posting guide! 
> > http://www.R-project.org/posting-guide.html
> > 
> 
> -- 
> The relationship between the computed price and reality is as 
> yet unknown.  
>                                              -- From the 
> pac(8) manual page
>