Error as.Date on Invalid Dates

Hi All,

I have an script in R which accepts user inputs for certain parameters,
particularly dates, which the user inputs as character strings.
eg:

 > date1 <- "2009-01-21"

The script later parses the input via the as.Date function:

as.Date(date1)

However, as.Date encounters an error when the string does not represent an
actual date.
eg:

 > date1 <- "2009-02-29"  # Note: 2009 not a leap year
as.Date(date1)

Error in fromchar(x) :
 character string is not in a standard unambiguous format

As I have many instances of date entries like this, date1, date2, date3,
etc. , I'd like the script to error out gracefully and to be able to point
the user to which date they need to correct, rather than "Error in
fromchar(x)...", which doesn't make it obvious what they need to do to fix
the error.

Ideally I'd love to send the user a message like:
print(paste(date1, "is an invalid date.  Refer to calendar.", sep=" "))

If anyone has any suggestions on catching this type of error and feedback
which directs the user, it would be much appreciated.

For reference, I am using a Windows Vista machine with:

R.version.string

[1] "R version 2.8.0 (2008-10-20)"

Thanks,
Brigid

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