[R] time zones and the chron to POSIXct conversion

[David Winsemius]

On Jun 16, 2012, at 1:01 AM, Jeff Newmiller wrote:

>> Why would a date function have
>> a TZ? `as.Date` wouldn't.
>
> This statement seems nonsensical to me.  POSIXt objects have tzone  
> attributes.

I earlier expressed surprise at this statement based on:

  attributes(as.POSIXct(chron('12/12/2000'), tz = 'UTC'))
$class
[1] "POSIXct" "POSIXt"

 > attributes(as.POSIXct(as.Date('12/12/2000', format="%m/%d/%Y")))
$class
[1] "POSIXct" "POSIXt"

I see now that it is one of those "sometimes they do" cases:

 > attributes(ISOdate(2001,12,24))
$class
[1] "POSIXct" "POSIXt"

$tzone
[1] "GMT"

-- 
David.

> Date and chron objects do not. Since they do not include tzone,  
> logically the user should be able to supply it during a conversion  
> from Date or chron to POSIXt.
>
> R is normally very good about distinguishing between a default value  
> such as zero (or "GMT") and undefined, and this "documented"  
> behavior breaks that consistency.
> ---------------------------------------------------------------------------
> Jeff Newmiller                        The     .....       .....  Go  
> Live...
> DCN:<jdnewmil at dcn.davis.ca.us>        Basics: ##.#.       ##.#.   
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> Playing
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> rocks...1k
> ---------------------------------------------------------------------------
> Sent from my phone. Please excuse my brevity.
>
> David Winsemius <dwinsemius at comcast.net> wrote:
>
>>
>> On Jun 15, 2012, at 11:18 PM, R. Michael Weylandt wrote:
>>
>>> It does seem that as.POSIXct.date doesn't respect the tz= argument
>> the
>>> generic suggests it would.
>>
>> Why would a date function have a TZ? `as.Date` wouldn't.
>>
>>> I'd think this is a bug that could be
>>> changed without breaking back-compatibility, but I don't have the
>>> power to make such things happen.
>>
>> What bug?
>>
>> Here is what ?chron says:
>>
>> "If x is character then it will be converted using as.POSIXct (with
>> the format argument, if any, passed to as.POSIXct) and tz = "GMT" and
>> then converted to chron. "
>>
>> So the call to `chron` below is using `as.POSIXct` first and then you
>> will be looking at differences in the printing conventions for  
>> POSIXct
>>
>> and POSIXlt objects.
>>
>> And then in the as.POSIX (lt and ct ) help pages we read:
>>
>> "Character input is first converted to class "POSIXlt" by strptime:
>> numeric input is first converted to "POSIXct". :
>>
>>> mode( chron('12/12/2000') )
>> [1] "numeric"
>>
>>>
>>> R-Core ruling?
>>
>> Er, not Core, just another useR.
>>
>>>
>>> Michael
>>>
>>> On Fri, Jun 15, 2012 at 2:25 AM, Jannis <bt_jannis at yahoo.de> wrote:
>>>> Hey R folks,
>>>>
>>>>
>>>> i found some strange (to me) behaviour with chron to POSIXct
>>>> conversion. The
>>>> two lines of code result in two different results, on ewith the
>>>> correct time
>>>> zone, one without:
>>>>
>>>> library(chron)
>>>> as.POSIXct(chron('12/12/2000'), tz = 'UTC')
>>
>>> format( as.POSIXct(chron('12/12/2000'), tz = 'UTC'), format="%Y-%m-
>> %d %H:%M %Z")
>> [1] "2000-12-11 19:00 EST"
>>
>>>> as.POSIXlt(chron('12/12/2000'), tz = 'UTC')
>>
>>> format( as.POSIXlt(chron('12/12/2000'), tz = 'UTC'), format="%Y-%m-
>> %d %H:%M %Z")
>> [1] "2000-12-12 00:00 UTC"
>>
>> These both look correct to me. (Different output conventions.)
>>
>>> as.POSIXct(chron('12/12/2000'), tz = 'UTC')
>> [1] "2000-12-11 19:00:00 EST"
>>> as.POSIXlt(chron('12/12/2000'), tz = 'UTC')
>> [1] "2000-12-12 UTC"
>>
>>
>>>>
>>>> Only the code below would give me a POSIXct object with the correct
>>
>>>> time
>>>> zone:
>>>>
>>>> as.POSIXct(as.POSIXlt(chron('12/12/2000'), tz = 'UTC'))
>>
>> I see the same output with that as I did with the second one:
>>
>>> as.POSIXct(as.POSIXlt(chron('12/12/2000'), tz = 'UTC'))
>> [1] "2000-12-12 UTC"
>>
>> If you want to force printing in a particular TZ then use format (or
>> strftime).
>>
>>> format(as.POSIXct(chron('12/12/2000'), tz = 'UTC'), tz="GMT",
>> format="%Y-%m-%d %Z")
>> [1] "2000-12-12 GMT"
>>
>>>>
>>>>
>>>> Is this a bug or desired behaviour?
>>
>> Jannis; You have not demonstrated what output you saw, so this is
>> still a guessing game.
>>
>>
>>
>> -- 
>>
>> David Winsemius, MD
>> West Hartford, CT
>>
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>

David Winsemius, MD
West Hartford, CT