[R] character to time problem

[John Kane]

I am trying to clean up some dates and I am clearly
doing something wrong.  I have laid out an example
that seems to show what is happening with the "real"
data.  The  coding is lousy but it looks like it
should have worked.

Can anyone suggest a) why I am getting that NA
appearing after the strptime() command and b) why the
NA is disappearing in the sort()? It happens with
na.rm=TRUE  and na.rm=FALSE
-------------------------------------------------
aa  <- data.frame( c("12/05/2001", " ", "30/02/1995",
NA, "14/02/2007", "M" ) )
names(aa)  <- "times"
aa[is.na(aa)] <- "M"
aa[aa==" "]  <- "M"
bb <- unlist(subset(aa, aa[,1] !="M"))
dates <- strptime(bb, "%d/%m/%Y")
dates
sort(dates)
--------------------------------------------------

Session Info
R version 2.4.1 (2006-12-18) 
i386-pc-mingw32 

locale:
LC_COLLATE=English_Canada.1252;
LC_CTYPE=English_Canada.1252;
LC_MONETARY=English_Canada.1252;
LC_NUMERIC=C;LC_TIME=English_Canada.1252

attached base packages:
[1] "stats"     "graphics"  "grDevices" "utils"    
"datasets"  "methods"   "base"     

other attached packages:
  gdata   Hmisc 
"2.3.1" "3.3-2" 

 (Yes I know I'm out of date but I don't like
upgrading just as I am finishing a project)

Thanks