[R] POSIXlt vs POSIXct

[Uwe Ligges]

On 29.03.2012 23:44, Steven R Corsi wrote:
> Thanks for this feedback. I was able to get that example to work, but I
> have gotten conflicting results with factors converted via POSIXct using
> the format= and tz= options to bring in excel-type dates and times. Here
> is what I have tried:
>
> dt.factor<- factor(c("3/17/2011 14:07","3/20/2011 7:18","3/22/2011
> 14:10","3/29/2011 9:07","7/15/2010 7:49"))
>
>>   dt.factor
> [1] 3/17/2011 14:07 3/20/2011 7:18  3/22/2011 14:10 3/29/2011 9:07  7/15/2010 7:49
> Levels: 3/17/2011 14:07 3/20/2011 7:18 3/22/2011 14:10 3/29/2011 9:07 7/15/2010 7:49
>
>
>>   as.POSIXct(dt.factor,format="%m/%d/%Y %H:%M",tz="CST6CDT")
> Error in as.POSIXlt.character(as.character(x)) :
>     character string is not in a standard unambiguous format


Works for me in R-release.

Uwe Ligges

>>   as.POSIXct(as.character(dt.factor),format="%m/%d/%Y %H:%M",tz="CST6CDT")
> [1] "2011-03-17 14:07:00 CDT" "2011-03-20 07:18:00 CDT" "2011-03-22 14:10:00 CDT"
> [4] "2011-03-29 09:07:00 CDT" "2010-07-15 07:49:00 CDT"
>
>
> So, it works only after converting the factor variable to character, but
> it is a simple matter to do that conversion.
>
> Thanks
> Steve
>
> On 3/29/2012 3:24 PM, Prof Brian Ripley wrote:
>> On 29/03/2012 21:05, Steven R Corsi wrote:
>>> That is a good tip. I have tried it and found that it works if I make
>>> sure it was converted to character first. strptime appears to accept
>>> factor variables which is the default when reading in the file
>>> originally.
>>
>> I would say that POSIXct is the way to go if you know the timezone of
>> the datetimes (it is essential to do the conversion).  But you don't
>> always do so, and then POSIXlt can be useful (although assuming UTC
>> can also work).
>>
>> Several times a month we get help requests about times on DST
>> transitions which shows that people too often think they know the
>> timezone and in fact do not (as the times are invalid or ambiguous in
>> the assumed timezone).
>>
>> as.POSIXct does work with factors ....
>>
>> z<- "2012-03-29 21:20:05"
>>> as.POSIXct(z)
>> [1] "2012-03-29 21:20:05 BST"
>>> as.POSIXct(factor(z))
>> [1] "2012-03-29 21:20:05 BST"
>>
>
>>> Thanks
>>> Steve
>>>
>>> On 3/29/2012 2:31 PM, MacQueen, Don wrote:
>>>> I also find that POSIXct is generally the most useful, and only use
>>>> POSIXlt in special cases.
>>>>
>>>> But have you considered as.POSIXct() instead of strptime()? It works
>>>> for
>>>> me, and I can't remember the last time I had to use strptime() for
>>>> converting character to date/time. (But I mostly don't work with
>>>> multiple
>>>> time zones, except for converting to/from UTC.
>>>>
>>>> -Don
>>>>
>>>
>>> ______________________________________________
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>>> 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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>
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