[R] Difference between times

[David Winsemius]

On Apr 18, 2014, at 12:59 PM, Prof Brian Ripley wrote:

> On 18/04/2014 19:46, Rui Barradas wrote:
>> Hello,
>> 
>> The reason why is that you've misspelled CET (not CEST)
> 
> Neither CET nor CEST are portable time-zone names.  We have not been given the 'at a minimum' information required by the posting guide, so please read ?Sys.timezone on your system.

Dear Prof;

Thanks for the impetus to yet again read that page. Despite frequently reading help pages and in particular reading that one many times, I still was not getting the 'tz' arguments correct on a Mac. I do now see that I was spelling my TZ incorrectly (as "Americas/Los_Angeles" rather than "America/Los_Angeles". 

Fellow Mac users may face a problem when using the Finder unless they set it up to display hidden ('dot') files. The /usr/ folder is "greyed out" but it still does open. If I restore my Finder defaults to not show system files and folders, I no longer see that directory and would not have been able to resolve my spelling error on my own:


> dt2 = as.POSIXct("2014-04-18 09.00", format="%Y-%m-%d %H.%M", tz = "America/New_York")
>  dt1 = as.POSIXct("2014-04-18 09.00", format="%Y-%m-%d %H.%M", tz = "America/Los_Angeles")
>  dt1-dt2
Time difference of 3 hours

I don't suppose a warning could be issued by the as.POSIXct code when a "tz" argument is not found in the database to let people know that 'UTC' will be the default?

-- 
David.


> 
> 
>> 
>> > dt1 = as.POSIXct("2014-04-18 09.00", format="%Y-%m-%d %H.%M", tz =
>> "CEST")
>> Warning messages:
>> 1: In strptime(x, format, tz = tz) : unknown timezone 'CEST'
>> 2: In as.POSIXct.POSIXlt(as.POSIXlt(x, tz, ...), tz, ...) :
>>   unknown timezone 'CEST'
>> > dt2 = as.POSIXct("2014-04-18 09.00", format="%Y-%m-%d %H.%M", tz =
>> "GMT")
>> > dt1 = as.POSIXct("2014-04-18 09.00", format="%Y-%m-%d %H.%M", tz =
>> "CET")
>> > dt1-dt2
>> Time difference of -2 hours
>> 
>> 
>> Hope this helps,
>> 
>> Rui Barradas
>> 
>> Em 18-04-2014 17:13, Nicola Sturaro Sommacal escreveu:
>>> Hi.
>>> 
>>> I am new to POSIX and I'd like to understand the reason of this
>>> difference.
>>> 
>>> dt1 = as.POSIXct("2014-03-29 09.00", format="%Y-%m-%d %H.%M")
>>> dt2 = as.POSIXct("2014-03-30 09.00", format="%Y-%m-%d %H.%M")
>>> dt2-dt1
>>> 
>>>> dt1[1] "2014-03-29 09:00:00 CET"> dt2[1] "2014-03-30 09:00:00 CEST">
>>>> dt2-dt1
>>> 
>>> Time difference of 23 hours
>>> 
>>> This is right, because on Mar 31 at 2 PM we jump directly to 3PM, DST.
>>> 
>>> On the contrary, I don't understand the following:
>>> 
>>> dt1 = as.POSIXct("2014-04-18 09.00", format="%Y-%m-%d %H.%M", tz =
>>> "CEST")
>>> dt2 = as.POSIXct("2014-04-18 09.00", format="%Y-%m-%d %H.%M", tz = "GMT")
>>> 
>>>> dt1[1] "2014-04-18 09:00:00 CEST"> dt2[1] "2014-04-18 09:00:00 GMT">
>>>> dt1-dt2Time difference of 0 secs
>>> 
>>> 
>>> I should expected a time difference of 2 hours, as CEST is GMT+2.
>>> 
>>> Anyone can help me?
>>> 
>>> Thank you,
>>> Nicola
>>> 
>>>    [[alternative HTML version deleted]]
>>> 
>>> ______________________________________________
>>> R-help at r-project.org mailing list
>>> https://stat.ethz.ch/mailman/listinfo/r-help
>>> PLEASE do read the posting guide
>>> http://www.R-project.org/posting-guide.html
>>> and provide commented, minimal, self-contained, reproducible code.
>>> 
>> 
>> ______________________________________________
>> R-help at r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide
>> http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
> 
> 
> -- 
> 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
> 
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

David Winsemius
Alameda, CA, USA