[R] date time problem

[Alemu Tadesse]

Thank you very much Jeff. Below is the data I used:
> Corrected_data
              SA_LST SA_GHI_mean
61759 3/11/2007 1:00     0.00000
67517 3/11/2007 2:00     0.00000
70017 3/11/2007 3:00     0.00000
70524 3/11/2007 4:00     0.00000
71061 3/11/2007 5:00     0.00000
71638 3/11/2007 6:00     0.00000
72113 3/11/2007 7:00    20.20873
72629 3/11/2007 8:00   123.14231
73274 3/11/2007 9:00   306.97343

>test1<-as.POSIXct(Corrected_data$SA_LST,format="%m/%d/%Y %H:%M",
tz="EST5EDT")
>test1
[1] "2007-03-11 01:00:00 EST" NA                        "2007-03-11
03:00:00 EDT" "2007-03-11 04:00:00 EDT"
[5] "2007-03-11 05:00:00 EDT" "2007-03-11 06:00:00 EDT" "2007-03-11
07:00:00 EDT" "2007-03-11 08:00:00 EDT"
[9] "2007-03-11 09:00:00 EDT"

> format(test1, tz="Etc/GMT", usetz=TRUE )
[1] "2007-03-11 06:00:00 GMT" NA                        "2007-03-11
07:00:00 GMT" "2007-03-11 08:00:00 GMT"
[5] "2007-03-11 09:00:00 GMT" "2007-03-11 10:00:00 GMT" "2007-03-11
11:00:00 GMT" "2007-03-11 12:00:00 GMT"
[9] "2007-03-11 13:00:00 GMT"
>

As you can see the "2007-03-11 02:00:00 EST" is not recognized and returned
as NA. This may be due to DST, which I do not know how to handle in R.

Best,

Alemu


On Sun, Dec 7, 2014 at 1:59 PM, Jeff Newmiller <jdnewmil at dcn.davis.ca.us>
wrote:

> You have not provided a reproducible example, so anything I say could be
> wrong just due to misinterpretation. Please read [1] for suggestions on
> making your examples reproducible, particularly regarding the use of dput
> to provide example data. You have also posted in HTML format, which can
> cause additional scrambling of communications on this list.
>
> From the notation you are using, I would guess that "Corrected_SA_data" is
> a data frame containing columns "date_time" and "TZ" at a minimum. The
> "date_time" column could be a vector of class POSIXlt or POSIXct... except
> that I cannot reproduce such an object that doesn't print some kind of
> timezone indicator in its output. What version of R are you using? Note
> that such an object contains a tz attribute already, so unless you have
> already made sure that the date_time column knows about that timezone, they
> are probably unrelated to each other.
>
> Note that any POSIXct object is internally represented as a specific
> instant of time. The tz attribute is only used to control how that time
> will be displayed. For example:
>
> testtime3 <- as.POSIXct( rawtime1, tz="America/New_York" )
> format( testtime3, tz="Etc/GMT", usetz=TRUE )
>
> If you want the output to omit the timezone information you can omit the
> usetz argument:
>
> format( testtime3, tz="Etc/GMT" )
>
> If you have not yet, you should read [2]. Note that three-letter timezone
> indicators (even though they are given in OUTPUT!) are at best unreliable
> for use in specifying timezones in R... read ?timezones.
>
> [1] http://stackoverflow.com/questions/5963269/how-to-make-
> a-great-r-reproducible-example
> [2] http://www.r-project.org/doc/Rnews/Rnews_2004-1.pdf
>
>
> On Sun, 7 Dec 2014, Alemu Tadesse wrote:
>
>  Dear R users
>>
>> I am puzzled by the following result from R script. I am trying to convert
>> local time to UTC time. Time zone is -5, therefore I used the following
>> approach.
>>
>> Below is the script.
>>
>>> Corrected_SA_data$date_time[k-1]
>>>
>> [1] "2007-03-11 01:00:00"
>>
>>> Corrected_SA_data$TZ[k-1]
>>>
>> [1] -5
>>
>>> Corrected_SA_data$date_time[k-1]-Corrected_SA_data$TZ[k-1]*3600
>>>
>> [1] "2007-03-11 07:00:00 MDT"
>>
>> I was expecting this last value to be something like "2007-03-11 06:00:00
>> UTC"
>>
>> Please correct me if I ma wrong.
>>
>> On the other hand I have
>>
>>> Corrected_SA_data$date_time[k]
>>>
>> [1] "2007-03-11 02:00:00"
>>
>>> Corrected_SA_data$TZ[k]
>>>
>> [1] -5
>>
>>> Corrected_SA_data$date_time[k]-Corrected_SA_data$TZ[k]*3600
>>>
>> [1] NA
>>
>> I am not sure why I am getting NA.
>>
>> Thank you for your help.
>>
>> Alemu
>>
>>         [[alternative HTML version deleted]]
>>
>> ______________________________________________
>> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
>> 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.
>>
>>
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