[R] Class for time of day?

[Gabor Grothendieck]

On Thu, May 21, 2009 at 5:04 PM, Stavros Macrakis <macrakis at alum.mit.edu> wrote:
> On Wed, May 20, 2009 at 12:28 PM, Gabor Grothendieck
> <ggrothendieck at gmail.com> wrote:
>>
>> There is a times class in the chron package.
>
> Perfect!  Just what I was looking for.
>
> On Wed, May 20, 2009 at 12:19 PM, jim holtman <jholtman at gmail.com> wrote:
>>
>> If you want the hours from a POSIXct, here is one way of doing it...
>> > y <- difftime(x, trunc(x, units='days'), units='hours')
>
> Ah, trunc.POSIXt -- I missed that one, thanks.
>
>> It depends on what type of computations you want to do with it.  You can
>> leave it as POSIXct and carry out a lot of them.  Can you specify what you
>> want?
>
> I am comparing irregular time series from different days, looking at the
> differences in intraday patterns.  So I want to put them on a common 0-24h
> scale and then do various kinds of plots and analyses, keeping the
> conventional display form (10:30 etc.) when specific times display or
> print.  It looks as though chron:::times combined with trunc.POSIXt pretty
> much solves my problem, except that `times` ignores the time units:

It uses hours/minutes/seconds for values < 1 day and uses days and fractions
of a day otherwise.

For values and operations that it has not considered it falls back to
the internal
representation.

Most of your examples start to make sense once you realize this.

>
>> as.POSIXct('2009-3-23 12:23')-trunc(as.POSIXct('2009-3-23 12:23'),"day")
> Time difference of 12.38333 hours
>> times(as.POSIXct('2009-3-23 12:23')-trunc(as.POSIXct('2009-3-23
>> 12:23'),"day"))
> Time in days:            <<< seems to treat difftimes as raw numbers!!
> [1] 12.38333
>
> Obviously I can work around this, but shouldn't `times` give an error when
> it encounters an object of unknown class rather than unsafely using its
> internal representation?  Of course, better still if `times` converted
> correctly....
>
> In general, `times` has other inconsistent and peculiar behavior:
>
> times(2) => Time in days: 2        Allows specifying multi-day periods, OK
> times(1.5) => Time in days: 1.5   Allows specifying fractional multi-day
> periods, OK
> times(0.5) => "12:00:00"   Inconsistent format compared to times(1.5)
> times("18:00:00") + times("18:00:00")  => Time in days: 1.5, OK
> times("36:00:00") => error      Why does it allow times(1.5) and
> times("18:00:00") + times("18:00:00") to specify 1.5 days, but not 36 hours?
> times(-0.5) => -0.5           Why doesn't it print Time in days: -0.5?
> times("18:00:00")/times("1:00:00") => Time in days: 18    Incorrect
> dimensions; meaningless result -- should be dimensionless
> times("18:00:00") * times("10:00:00") => 07:30:00     Incorrect dimensions;
> meaningless result.
> sin(times("18:00:00")) => 16:21:34     Meaningless result -- should be error
>
> It's nice that R has a class system, but if code ignores the class....
>
>> There is an article on dates and times in R News 4/1.
>
> Thanks for the pointer.
>
>           -s
>
>