[R] Applying strptime() to a data set or array

[Andrew Athan]

I'm sure this is just the result of a basic misunderstanding of the 
syntax of R, but I am stumped.

A <- 
read.table(file="sumByThirtyMinute.csv",sep=",",col.names=c("date","pandl"))

A now consists of thousands of rows, but A$date is a string...
...
3183 2006-02-28 12:00:00    548.470
3184 2006-02-28 12:30:00    515.240
3185 2006-02-28 13:00:00    140.120
3186 2006-02-28 13:30:00    450.940
3187 2006-02-28 14:00:00    318.570
...




So, I try to convert A$date to a POSIXlt ...

A[,1]<-strptime(A$date,"%Y-%m-%d %H:%M:%s")
A$date<-strptime(A$date,"%Y-%m-%d %H:%M:%s")


which gives me a warning that the length of the array I am trying to 
replace A$date with is 9 ... but if I print strptime(A$date,"%Y-%m-%d 
%H:%M:%s"), it clearly has thousands of rows.  Yet, if I ask for 
length(strptime(A$date,"%Y-%m-%d %H:%M:%s")), I get 9.

What am I doing wrong?  Do I need to convert the return value of 
strptime(A$date,"%Y-%m-%d %H:%M:%s") to some array/vector/matrix 
datatype before attempting to assign it?

Thanks,
Andrew