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]] > > ______________________________________________ > R-devel@r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-devel > -- Felix Andrews / 安福立 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 M: +61 410 400 963 T: + 61 2 6125 4670 E: felix.andr...@anu.edu.au CRICOS Provider No. 00120C -- http://www.neurofractal.org/felix/ ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel