[R] Problems with as.POSIXct

[KENNETH R CABRERA]

Hi R users:

This is a very strange problem:

Why this instruction shows me NA?, 
and any other date shows me that error!

as.POSIXct(strptime("1992-5-3",format="%Y-%m-%d"))

This is my R version on windows 7.
"R version 2.13.1 Patched (2011-08-25 r56794)"

Thank you for your help.