From felix at nfrac.org  Thu May 20 04:48:26 2010
From: felix at nfrac.org (Felix Andrews)
Date: Thu, 20 May 2010 12:48:26 +1000
Subject: [Rd] pretty.Date(): new "halfmonth" time step
In-Reply-To: <AANLkTim1YYlrryyI5-24VgYNVBTU-fxNe8cZlowrL86e@mail.gmail.com>
References: <AANLkTim1YYlrryyI5-24VgYNVBTU-fxNe8cZlowrL86e@mail.gmail.com>
Message-ID: <AANLkTil4POsPLZ40AKT9Td9sO7HFyPTw5PynrT9WxDSa@mail.gmail.com>

On 20 May 2010 11:56, Daniel Murphy <chiefmurphy at 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 / ???
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Integrated Catchment Assessment and Management (iCAM) Centre
Fenner School of Environment and Society [Bldg 48a]
The Australian National University
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M: +61 410 400 963
T: + 61 2 6125 4670
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[Rd] pretty.Date(): new "halfmonth" time step
Felix Andrews