[R-SIG-Finance] N'th of month working day problem

[Research]

Dear all,

Some time ago I received some very kind  help (special thanks to Gabor) 
to construct a function that isolates the n'th   working day of each 
month for zoo object (time series) to create monthly data from daily 
observations.

I found out that the code works fine except for the 29 till 31st dates 
of each month as it skips some months (February for example).

If  you could help me isolate the problem I would be grateful as I can 
not find a way to explain to R to keep the last  working day of month if 
I choose the 29th, 30th or 31st dates...

I enclose  a working version of the function and a script for demo purposes.

Many thanks in advance,
Costas

library(fImport)
library(zoo)
DJ<-yahooSeries("^DJI", frequency="daily", nDaysBack=10000)
DJd<-as.zoo(DJ[,4])

### Choose number of day for month

chooseday<-function(z, day)

     {

     # z.na is same as z but with missing days added using NAs
     # Its formed by merging z with a zoo-width series containing all days.

     rng <- range(time(z))
     z.na <- merge(z, zoo(, seq(rng[1], rng[2], by = "day")))

     # form a series that has NAs wherever z.na does but has 1, 2, 3, ...
     # instead of z.na's data values and then use na.locf to fill in NAs

     idx <- na.locf(seq_along(z.na) + (0 * z.na))

     # pick off elements of z.na corresponding to i'th of month

     noofday <- paste(day)

     if (day<10) noofday<-paste("0",day, sep="")

     tempdata<-z.na[idx[format(time(z.na), "%d") == noofday]]

     return(tempdata)

     }

length(chooseday(DJd,1))
length(chooseday(DJd,2))
length(chooseday(DJd,31))
length(chooseday(DJd,30))
length(chooseday(DJd,29))
length(chooseday(DJd,28))
tail(chooseday(DJd,31))