pretty.Date(): new "halfmonth" time step

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

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