# [R] how to calculate time offset between timezones?

Armstrong, Whit whit.armstrong at highbridge.com
Thu Jan 10 17:02:07 CET 2008

```well, strangely,  ISOdatetime(2008,1,1,1,0,0,tz="AST") creates a UTC
timezone date on my system:

> ISOdatetime(2008,1,1,1,0,0,tz="AST")
[1] "2008-01-01 01:00:00 UTC"
>

and if you compare the numeric values of the UTC datetime and the GMT
datetime, the are definitely the same:

> as.numeric(ISOdatetime(2008,1,1,1,0,0,tz="AST"))
[1] 1199149200
> as.numeric(ISOdatetime(2008,1,1,1,0,0,tz="GMT"))
[1] 1199149200
>

I think the failure to recognize AST may have something to with the
timzones listed in /usr/share/zoneinfo (on Ubuntu linux) where I found
AST4:

whit at spartan:~\$ find /usr/share/zoneinfo -name "AST*"
/usr/share/zoneinfo/SystemV/AST4
/usr/share/zoneinfo/right/SystemV/AST4
/usr/share/zoneinfo/posix/SystemV/AST4

and when I use it as the tz argument:

> ISOdatetime(2008,1,1,1,0,0,tz="AST4")
[1] "2008-01-01 01:00:00 AST"
>

and

> ISOdatetime(2008,1,1,1,0,0,tz="AST4") -
ISOdatetime(2008,1,1,1,0,0,tz="GMT")
Time difference of 4 hours
>

-Whit

> -----Original Message-----
> From: r-help-bounces at r-project.org
> [mailto:r-help-bounces at r-project.org] On Behalf Of dankelley
> Sent: Thursday, January 10, 2008 10:21 AM
> To: r-help at r-project.org
> Subject: [R] how to calculate time offset between timezones?
>
>
> #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.
> 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!
> --
> View this message in context:
> http://www.nabble.com/how-to-calculate-time-offset-between-tim
> ezones--tp14736453p14736453.html
> Sent from the R help mailing list archive at Nabble.com.
>
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