[R] Problem with GMT+/- time zones

[Andrew Digby]

Wow. Thanks very much for pointing that out - I never would have guessed it was deliberate that + and - were reversed!

For future reference for anyone else similarly confused by this departure from time zone and mathematic convention, here's the relevant part of en.wikipedia.org/wiki/Tz_database:

"The special area of Etc is used for some administrative zones, particularly for "Etc/UTC" which represents Coordinated Universal Time. In order to conform with the POSIX style, those zone names beginning with "Etc/GMT" have their sign reversed from what most people expect. In this style, zones west of GMT have a positive sign and those east have a negative sign in their name (e.g "Etc/GMT-14" is 14 hours ahead/east of GMT.)"

Thanks,

Andrew


On 2/02/2012, at 20:46 , Jeff Newmiller wrote:

> Should has nothing to do with it. That is the way the Olsen tz database works.  See en.wikipedia.org/wiki/Tz_database.
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> 
> Andrew Digby <andrewdigby at mac.com> wrote:
> 
>> 
>> I'm struggling with time zone version when expressed as hours offset
>> from GMT. Can anyone confirm that the behaviour below is incorrect? It
>> seems that the GMT offsets are backwards:
>> 
>>> format(as.POSIXct("2011-05-23 17:23:00",
>> tz="Europe/London"),tz="America/New_York",usetz=T)
>> [1] "2011-05-23 12:23:00 EDT"
>> 
>> - this works.
>> 
>>> format(as.POSIXct("2011-05-23 17:23:00",tz="GMT"),tz="GMT-5",usetz=T)
>> [1] "2011-05-23 22:23:00 GMT"
>> 
>> - this doesn't work: 17:23:00 GMT should be 12:23:00 GMT-5!
>> 
>> 
>> Thanks.
>> 
>> 
>> R version 2.13.0 (2011-04-13)
>> Platform: x86_64-apple-darwin9.8.0/x86_64 (64-bit)
>> 
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