[R] time zones and the chron to POSIXct conversion

[R. Michael Weylandt]

On Fri, Jun 15, 2012 at 11:26 PM, 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.
>

It seems that as.POSIXct.date and as.POSIXct.Date are different things
here. I know that Date (capital) objects don't care tz properties --
I'm less clear on what comprises a "date" object here. There's also
as.POSIXct.dates to be thrown in...

>
>> 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"
>
>>

"Bug" was admittedly too strong a word. Infelicity perhaps?

? as.POSIXct also says this:

      tz: A timezone specification to be used for the conversion, _if
          one is required_.  System-specific (see time zones), but ‘""’
          is the current timezone, and ‘"GMT"’ is UTC (Universal Time,
          Coordinated).

So R-Core doesn't have a problem with it being ignored. What I'm not
clear on is what exactly that means -- is the tz= argument a property
of the input or the output? Looking around, I can't see a super clear
pattern:

as.POSIXct.Date, as.POSIXct.date, and as.POSIXct.dates all ignore it
as.POSIXct.POSIXlt seems to pass it to .POSIXct which associates it
with the output
as.POSIXct.numeric seems to pass it to as.POSIXct but alongside the
origin (which is a sort of input)
ISOdatetime suggests it's both in and out.

It seems contrary to WRE 7.0 which says "Only add parameters and
defaults to the generic if they make sense in all possible methods
implementing it." I see some reason to each choice individually, but
the sum total doesn't emerge so naturally. My intuition for the
[Dd]ate(s) would have been something like what Jeff expressed
upthread.

Now that I've said my peace and looked at it more, I think I do
retract my earlier objection: my assumptions about what the tz=
argument would do were incorrect and I had not properly checked them
against the documentation and the source code.

My apologies to all for the noise and my continued thanks to R-Core
for their great gift to our little corner of the computing world.

Michael

>> 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
>