Weird date conversion

I am a new R user, but have searched the manuals on the topic , but found no
suitable solution.
I have imported a 20 years? time-series data from csv format into R.  The
date variable was a factor and I used the command ?  x$date <-
as.Date(date,format= "%d%b%Y") ? to convert to a date format. The conversion
worked for most dates, but all data that corresponds to the months of May
and October for all years returned NAs. Strangely, I had experienced similar
problem with a different data set on another computer using earlier version
of R  some time ago.  The computer that I use now is a brand new
installation of R and other software.

I am running R version 2.14.2 on Windows 7. 
I would appreciate if someone would help on this recurring problem.

Thanks, 



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