Felix,
I see your point about the "halfway" point acting like the first day of the
month in its relativity to the ending date of the month: they're both
variable.
I'm an actuary. Like accountants, actuaries tend to measure "financial" time
in months because of how the books close. But in our mathematical models
time takes on a continuous nature. So our problem is how to build a computer
representation of time that reflects both its continuous nature as well as
the varying-length, "discrete" nature of financial months. I've found that a
fixed value for a halfway point complicates actuarial calculations. It could
serve other purposes just fine, however.
Best regards,
Dan
On Wed, May 19, 2010 at 7:48 PM, Felix Andrews <fe...@nfrac.org> wrote:
> On 20 May 2010 11:56, Daniel Murphy <chiefmur...@gmail.com> wrote:
> >>Much better to implement directly what this is trying to do: i.e. to
> >>have a "halfmonth" time step. This is just the union of two "monthly"
> >>sequences, one on the 1st of each month and another on the 15th of
> >>each month.
> >
> > For some applications that might be true. But not for others. For a month
> > with 31 days, there are 14 days before the 15th of the month and 16 days
> > after the 15th, so, for example, March 16th (specifically noon) rather
> than
> > March 15th would be the halfway point if you define "halfway" in terms of
> > the distances to the beginning and end of the month. For a month with 30
> > days -- like April -- the halfway point would be the instant between the
> > 15th and the 16th of the month. Do you label that instant April 15 or
> April
> > 16? (I prefer "15".) Don't get me started on February.
>
> Dan, you are correct: the midpoint of a 30 day month is the 16th at
> 00:00. That instant is called the 16th according to print.POSIXt.
>
> junstart <- as.POSIXct("2000-06-01 00:00", tz="GMT")
> julstart <- as.POSIXct("2000-07-01 00:00", tz="GMT")
> junstart + ((julstart - junstart) / 2)
> #[1] "2000-06-16 GMT"
>
> How embarassing...
> So I think it would be better to use 16 rather than 15 for the
> "halfmonth" time step.
>
> Yes, months have variable lengths, but I think it is best to use a
> consistent date (the 16th) than to calculate exact midpoints, just as
> a normal monthly series has a consistent date (the 1st) and has
> variable lengths.
>
> Regards
> -Felix
>
>
> >
> > - Dan Murphy
> >
> > [[alternative HTML version deleted]]
> >
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> >
>
>
>
> --
> Felix Andrews / å®ç¦ç«
> Postdoctoral Fellow
> Integrated Catchment Assessment and Management (iCAM) Centre
> Fenner School of Environment and Society [Bldg 48a]
> The Australian National University
> Canberra ACT 0200 Australia
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>
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