problem with unlist POSIX date at midnight

as.character(acc.period[16])

Dear R-users,

I use unlist of POSIX dates to extract the year, hour etc. With that I
can search for files in my database which are in the form
'yyyymmddhh_synops.txt'
However, I get stucked during midnight where unlist just gives NA's.
The script is given below; the problem accurs at acc.period[16]
(midnight). However when I write out the character, unlist works well.
But as.character(acc.period[16]) is not the solution....

dates=seq(as.Date("2006-03-01"), as.Date("2006-11-01"), "days")
i=1
begin=paste(dates[i]-1,"09:00:00")
end=paste(dates[i],"08:00:00")
acc.period=seq(as.POSIXct(begin),as.POSIXct(end),"hour")

unlist(strptime(acc.period[15],format="%Y-%m-%d %H:%M:%S"))

 sec   min  hour  mday   mon  year  wday  yday isdst
   0     0    23    28     1   106     2    58     0

unlist(strptime(acc.period[16],format="%Y-%m-%d %H:%M:%S"))

 sec   min  hour  mday   mon  year  wday  yday isdst
  NA    NA    NA    NA    NA    NA    NA    NA    -1

unclass(strptime(acc.period[16],format="%Y-%m-%d %H:%M:%S"))

$sec
[1] NA

$min
[1] NA

$hour
[1] NA

$mday
[1] NA

$mon
[1] NA

$year
[1] NA

$wday
[1] NA

$yday
[1] NA

$isdst
[1] -1

a="2006-03-01 00:00:00"
unlist(strptime(a,format="%Y-%m-%d %H:%M:%S"))

 sec   min  hour  mday   mon  year  wday  yday isdst
   0     0     0     1     2   106     3    59     0

sessionInfo()

R version 2.4.0 (2006-10-03)
i386-pc-mingw32

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

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

Thanks in advance!

Hanneke