[R] how to calculate time offset between timezones?

[Prof Brian Ripley]

On Thu, 10 Jan 2008, dankelley wrote:

>
> #QUESTION#
>
> Is there a way to calculate the offset between timezones, e.g. from "AST" to
> "GMT"?
>
> #DETAILS#
>
> I am working with data files that use local time, and that indicate the
> timezone by e.g. "AST" (Atlantic Standard Time, my local time) in a header.

The problem is that AST is not a valid timezone (at least on my Mac), so 
is being ignored.  I think you want "America/Halifax".  (Works for me on 
Leopard as well as on Linux.)

The documentation for R-devel may help here:

https://svn.r-project.org/R/trunk/src/library/base/man/timezones.Rd

But earlier versions do say things like

      If a timezone is needed and that specified is invalid on your
      system, what happens is system-specific but it will probably be
      ignored.

> I was guessing that
>
>> ISOdatetime(2008,1,1,1,0,0,tz="GMT") -
>> ISOdatetime(2008,1,1,1,0,0,tz="AST")
>
> would work, but it gives a difference of 0.  However, when I do
>
>> ISOdatetime(2008,1,1,1,0,0,tz="GMT") - ISOdatetime(2008,1,1,1,0,0)
>
> I get a -4h offset, which is correct.  The odd thing is, I am in AST time,
> as shown by the following.
>
>> ISOdatetime(2008,1,1,1,0,0)
> [1] "2008-01-01 01:00:00 AST"
>
> #SYSTEM#
>
> Mac OS X system, with R 2.6.1
>
> #PS#
>
> I am aware that this sort of thing is system dependent, and so there may not
> be a general solution.  But even if I can only get it working on my own
> particular system, I'd be happy!
>

-- 
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